P0301 Code in 2026: Cylinder 1 Misfire, the Free Swap Test, and What Your Badge Changes

P0301 is the misfire code that names a cylinder, and that one fact changes the diagnosis. A misfire the computer cannot place is a question about what every cylinder shares; a misfire it can place is a question about one cylinder's own parts — and about whether that cylinder is still sealing.

What it is not is a verdict on a coil. Across 100 P0301 complaints we read end to end, ignition parts are the reported action in 5. Coolant reaching a combustion chamber accounts for 29, and worn piston rings fouling one plug for 14.

By the end of this guide you will know which of five routes your P0301 belongs to, the three tests that settle it, and what your make changes — because in this file the badge on the grille predicts the cause better than anything else. Claims marked 🟢 rest on a document in Sources, opened and read; ⚪ marks estimates and owner reports.

I built Pulscar — an AI tool that reads car problems from a 30-second recording — after a $380 "full diagnostic" in Phuket in 2025 led to roughly $6,000 over six months across several shops, and the sound never went away (the story is on the About page).

How to use this guide

Every route is defined by what the tests show and what else is disappearing from the engine, not by the code.

One rule overrides everything: a named cylinder tells you where to test, not what to buy. Honda writes the same discipline into its own procedure: if P0301 is stored, pull the plug from cylinder 1 and look at it, and if that plug is not fouled, "This bulletin does not apply."

First: what naming one cylinder rules in, and what it rules out

Write down every code stored with it, including pending and history codes, because the company P0301 keeps decides the route.

What one cylinder rules in. Everything that belongs only to that cylinder: its plug, its coil, its injector, its valves and valve gear, its rings and the seal of that combustion chamber.

What it rules out, and only weakly. A perfectly shared cause — a fuel pump, an air leak into the plenum, a lean condition — usually shows up as P0300 or as several cylinder codes at once. But shared causes are not evenly shared. One cylinder is always slightly worse, and when the count crosses the threshold there first, you get P0301 on a car whose problem is not cylinder 1 at all. That is why 39.7 percent of the complaints naming P0301 also mention coolant.

What it never tells you. Whether the trouble is spark, fuel or compression.

Is cylinder 1 special? What the whole code family shows

The search term you typed is arbitrary, and it is worth ten seconds to prove it.

We counted every complaint in the federal file that names each cylinder misfire code as a whole word, and every manufacturer document that does the same.

"Lone-cylinder" means the report names that code and no other cylinder code and not P0300 — the cleanest version of the question this page answers.

Cylinder 1 is not the leader on either measure. P0302 is, on both. Manufacturer documents agree: 504 distinct NHTSA documents name P0304 and 480 name P0301. The folk explanation — that cylinder 1 sits nearest the timing drive, or the front of the engine, or the coolant inlet, and therefore fails first — is not visible in the public record.

Cylinders 5 to 8 are rare for a boring reason. Most vehicles in this file have four cylinders, so there is no cylinder 5 to fail.

The practical consequence. If your code is P0303, everything on this page applies with the number changed. And if a shop tells you cylinder 1 is a known weak spot on your engine, ask which document says so.

Where cylinder 1 actually is

Two minutes here prevents an expensive mistake, because cylinder 1 is a position on an engineering drawing, not a position in the engine bay.

On an inline engine cylinder 1 is at one end, usually the end furthest from the transmission — but not always, and on a transverse installation that end may be on either side of the car.

On a V6 or V8 cylinder 1 defines bank 1, and bank 1 is frequently the rear bank on a transverse V6. Honda's own V6 documents describe the rear bank that way, which is how an owner spends an afternoon working on the wrong three cylinders.

On a V6, cylinders 1 and 4 are usually companions, arriving at top dead centre together. A fault in the part that tells the computer where the crankshaft is produces misfire counts on both, and neither cylinder is broken. Take the numbering from the service information for your exact engine, not from a forum picture.

The 15-minute driveway protocol

1. Read every code, pending and history, and save the freeze frame. A $20 to $40 reader gives you the full list plus the engine temperature, load and fuel trim when the code set. Clearing throws that away, and some warranty extensions can only be claimed while the code is present.
2. Note whether the light is steady or flashing at idle. Flashing means the converter is at risk; go gently.
3. Check the coolant level cold and mark it with tape. A level that falls over three mornings with no puddle changes the whole route — the most important two minutes on a Ford EcoBoost.
4. Pull the plug from the named cylinder and look at it. Wet with fuel, black and oily, cracked, or soaked in coolant residue — each points somewhere different. Honda's own procedure stops here if the plug is clean.
5. Do the coil swap test. Move the coil from cylinder 1 to a neighbour, clear the misfire counters, drive the same conditions. If the misfire follows the coil, you have your answer for the price of two bolts.
6. Watch the per-cylinder misfire counters. Most $30 readers show them, and a counter that climbs only on cold starts or only under load is evidence no shop can get from a stored code.
7. Run the VIN for recalls, extensions and campaigns, and ask a dealer for the ones the federal lookup cannot show.

Fifteen minutes and a cheap reader give you most of what the first half of an $80 to $200 diagnostic fee buys. What a diagnostic should cost covers the rest.

Find your situation: eight ways people arrive here

"I replaced the plug and coil and the code came back." → Routes 3, 4 and 5. In our file, 54 of the 85 reports that mention fitting ignition parts also say the fault returned.

"Coolant keeps going down and there is white smoke." → Route 4, today. On a 2014-2019 Fusion, 2017-2019 Escape, 2015-2018 Edge, MKC or MKZ, that is the documented branch.

"2008-2013 Honda Accord, Odyssey, Pilot or Crosstour, one fouled plug." → Route 2. A class-action settlement extended the powertrain warranty for this exact code range.

"Only on a cold morning, fine once warm." → Route 5. Toyota, Mazda, Nissan and Subaru all publish cold-start misfire documents.

"P0301 and P0304 together on a V6." → Route 5. Companion cylinders point at the crank position signal, not at two cylinders.

"P0301 with a rich code, and the injector was mentioned." → Route 3.

"There is a knock or a tick with it." → Route 4, and shut the engine off. The knocking guide covers the noise.

"The light is flashing." → The flashing light guide walks the roadside hour, then come back here.

The price ladder: read it from the bottom up

Read it from the bottom up. The top three lines are free, and they are the ones that decide whether the bottom two ever happen.

P0301 by make: the badge changes the answer

This is the finding that surprised us most, and it is the reason people type the make into the search box after the code.

We split the 315 complaints by manufacturer group and counted the same vocabulary in each. The result is not a small difference. It is two different faults wearing one code.

Ford, Lincoln and Mercury — 137 reports, 43.5 percent of the file. Coolant or a head gasket appears in 84.7 percent, and 48.9 percent end in an engine or block replacement. Not one of the 137 mentions a piston ring. Models: Escape 54, Edge 36, Fusion 25; model years 2017 and 2018 alone are 142 of the whole 315. Ford's bulletins for the 1.5L and 2.0L EcoBoost list P0301 beside P0300, P0316, P0217, P1285 and P1299 and send the technician to a pressure test, a borescope and a short or long block. Our P0300 guide covers that family in full; do not let a shop sell you a coil before that pressure test.

Honda and Acura — 81 reports, 25.7 percent. The picture inverts. Piston or oil rings are named in 25.9 percent, injectors in 28.4 percent, plugs in 43.2 percent, and coolant in 3.7 percent. Only 7.4 percent end in an engine. And 60.5 percent name a recall, extension or settlement, the highest share of any group — because Honda has published more coverage for this code than anyone. Models: Odyssey 28, Accord 18, CR-V 16, Pilot 10.

Chevrolet and GMC — 18 reports, too few for a share, so here are the counts: 5 name a valve, lifter or camshaft, 5 compression, 5 a coil, 5 a plug. GM's own documents point the same way — oil-fouled plugs, lifters, compression — rather than at coolant. Toyota and Lexus — 10 reports, also too few to carry a percentage; what Toyota publishes is more useful than what ten owners typed, and it is in Route 5. Hyundai 20 and Kia 19: coils 35 percent and plugs 30 percent on the Hyundai side, while on the Kia side rings and engine replacements appear more often than coils.

How to use this. The make does not tell you the answer. It tells you which test to run first: a cooling system pressure test on a Ford EcoBoost, a look at the plug on that cylinder on a Honda V6, a compression reading on a GM V8. Counts across makes reflect very different sales volumes and nothing here ranks reliability.

The five routes, cheapest first

Route 1 · That cylinder's own spark — the plug, the coil and the free swap test

The tell. A steady stumble that is worse under load, often right after plug work, a car wash or heavy rain, with no coolant loss and no smoke.

What it actually is. A spark that is weak, leaks to ground, or never arrives at that one cylinder. Mazda's 01-014-16-3208A, for 2013-2016 Mazda3, Mazda6 and CX-5 models, blames a rough idle and lack of power with P0300 through P0304 on "an internal failure of the ignition coil(s) due to a high difference between cold and warm ambient temperature" — a fault that looks seasonal and intermittent. GM's PIP4525D, for 2008-2009 full-size trucks and SUVs, pairs a misfire with oil consumption, names P0300 and P0301, and tells the technician to "inspect all of the spark plugs for oil" first.

The cheap cause that gets skipped. The plug itself, read rather than replaced. A plug that comes out black and oily is a message from below the head. A plug that comes out wet with fuel says the spark failed, not the fuel. A plug that comes out clean, on a car with a stored P0301, is Honda's own instruction to stop and troubleshoot elsewhere.

Fix it yourself — the coil swap test, in order. Engine off and cool. Note which cylinder the code names. Move that coil to a neighbouring cylinder and move the neighbour's coil into its place. Clear only the misfire counters if your reader allows, and drive the same conditions that set the code. Misfire moves with the coil: the coil is the part, $150 to $400 installed. Misfire stays on cylinder 1: the coil is innocent; pull the plug and look at it. Misfire goes random: you are in the shared-cause world of P0300. Then repeat the same logic with the plug. The honest limit: never unplug a coil with the engine running, torque plugs to specification on aluminum heads, and a rear-bank V6 plug that needs the intake removed is shop work. The spark plug cost guide prices it.

At the shop — say this: "P0301. Before any parts, swap the coil to another cylinder and tell me whether the misfire followed it, and show me the plug you pulled from cylinder 1."

Route 2 · Your VIN is on a document — extensions, settlements and campaigns

The tell. Your car is in the table below, or the code has been stored long enough for someone to mention a lawsuit.

What it actually is: Honda's settlement, in the manufacturer's own words. Service bulletin 13-081, version 19 of 8 October 2020, for 2011-2013 Odyssey models, states that "the piston rings on certain cylinders may rotate and align, which can lead to spark plug fouling," and that this "can set DTCs P0301 No. 1 cylinder misfire detected, P0302 No. 2 cylinder misfire detected," and so on to P0304. The remedy cleans the pistons and replaces the rings and the affected plugs, at 8.5 flat-rate hours. The coverage: "American Honda is extending the powertrain warranty to cover repairs related to engine misfire (that triggers DTCs P0301 through P0304) to 8 years with unlimited mileage from the original date of purchase and has settled a class action based on this remedial measure." Sister bulletins cover the 2008-2012 Accord, 2010-2012 Crosstour, 2008-2010 Odyssey and 2009-2013 Pilot.

The cheap cause that gets skipped. The diagnostic gate, which is free. Honda's flowchart is two questions: is one of P0301 to P0304 stored, and "Remove and check the spark plug(s) on the affected cylinders (example: if P0301 is stored, check the spark plug on cylinder No. 1). Are the spark plug(s) fouled?" If the answer is no, "Stop. This bulletin does not apply." That means the paperwork for an 8.5-hour engine job turns on one plug that anyone can look at.

Two limits, stated plainly. The extension "does not apply to any vehicle that has ever been declared a total loss or sold for salvage." And it counts from the original purchase date, so on a 2013 vehicle the term has run out — owners in our file describe exactly that, including one refused the work because a previous shop had cleared the codes.

At the shop — say this: "Run this VIN for every recall, emission recall, warranty extension, special coverage and customer satisfaction program, open or completed, and check bulletin 13-081 if this is a Honda V6. Do not clear the code before you check."

How do you hear a single-cylinder misfire, and tell it from the other things that shake a car? Stand at the tailpipe with the engine idling. A healthy engine sends an even stream of exhaust pulses. One cylinder that is not firing leaves a gap in that stream, so the note turns into a regular puff, chuff or stumble that keeps perfect time with engine speed and speeds up as you raise the revs. A random misfire is less regular, a lope that comes and goes. A metallic tick from the top of the engine points at the valvetrain, and a deep knock that rises with revs is a stop sign, not a diagnosis. Film 30 seconds at the open hood with the engine idling and Pulscar's free check returns a likely cause, a severity and a repair-cost estimate, or "inconclusive" — it does not decide whether the car is safe to drive; a flashing light does. Free, no card.

Route 3 · The injector on that cylinder — and the test that proves it

The tell. P0301 with a rich or lean code, a fuel smell, a plug that comes out soaked, or fuel in the oil.

What it actually is. A single injector that leaks, sticks shut, or has stopped atomising. The named cylinder is doing you a favour here, because injector faults are one of the few problems that really do live on one cylinder. Mazda's 01-017-16-3242 makes the link explicit: injector circuit codes P0201 to P0204 come with a rough-running engine and "engine misfire related DTCs (P0301, P0302, P0303, P0304) are usually stored at the same time." In our complaint file the Honda group names an injector in 28.4 percent of P0301 reports, against 0.7 percent in the Ford group — the sharpest single split in the data.

The cheap cause that gets skipped. The data the shop already has. An injector balance or contribution test tells you how much each cylinder is actually contributing, and a fuel trim reading tells you whether the mixture on that bank is rich or lean. One owner in our file recorded a dealer diagnosis of a "cylinder 1 internal injector failure" found that way, quoted at $526. Another watched a shop replace the injectors, the turbo and the plugs in sequence before finding a head gasket.

Fix it yourself — the second swap test. On engines where the injectors are accessible and the fuel rail does not have to come off under pressure, injectors can be moved between cylinders exactly like coils, and the misfire either follows or it does not. The honest limit: direct-injection injectors sit under high pressure and often need new seals and a torque procedure, and on many modern engines a replaced injector must be coded to the computer. If your fuel rail is not obviously accessible with the engine cold and depressurised, this is shop work. The fuel injector cost guide prices it.

At the shop — say this: "P0301. Show me the injector balance or contribution figures and the fuel trims for that bank before you quote an injector, and tell me whether the plug on that cylinder was wet."

Route 4 · The cylinder is not sealing — compression, coolant and rings

The tell. Coolant that goes down with no puddle. White smoke that lingers, or a sweet smell. Blue smoke and oil use. A knock or a tick. A misfire that is worst after the car has sat. And a plug from that one cylinder that looks nothing like its neighbours.

What it actually is. The combustion chamber is no longer sealed, or something is getting in that will not burn. Coolant is the largest version — 39.7 percent of all 315 reports mention it, rising to 84.7 percent in the Ford group. Oil is second: Honda's rings rotate until their gaps line up, oil passes, and that cylinder's plug fouls. Mechanical is third: GM's 23-NA-043 replaces valve lifters and guides on 2020-2021 5.3L and 6.2L trucks and SUVs for misfire codes including P0301, and Toyota's T-SB-0010-24 Rev2 replaces all eight exhaust-side valve lash adjusters on 2018-2025 Camry, 2019-2025 Corolla and RAV4, 2022-2025 Corolla Cross and 2021-2022 Avalon models for codes that appear "upon cold soak start up with engine coolant temperatures between 14°F – 41°F" — 2.8 to 3.0 hours, covered for 60 months or 60,000 miles.

The cheap cause that gets skipped. The compression test. It costs $100 to $300, it produces a number rather than an opinion, and only 20 of our 315 complaints mention one at all. The reports that do mention it read very differently from the ones that do not. A Ram 1500 owner changed plugs, a coil pack, injector cleaner and an intake sensor, then ran a compression check and found cylinder 1 had lost compression and a valve "was barely moving." A 2023 Escape owner recorded the modern version: "Ran misfire monitor & Relative compression and cylinder 1 is the issue with 70 PSI. Performed cylinder head leak down test — exhaust valve leaking."

Fix it yourself — measure the loss, do not mask it. Engine cold, cap never opened hot. Mark the coolant reservoir with tape each morning for three days and write the mileage beside it. Look at the tailpipe on the first start for white smoke that lingers after the exhaust is warm. Pull the plug from the named cylinder and compare it side by side with one from a neighbour. The honest limit: no sealer in a bottle, and no driving on a falling coolant level. Compression, leak-down and borescope work need tools and a warm, safe engine. The head gasket cost guide and the white smoke guide cover what comes next.

At the shop — say this: "P0301 with coolant loss. Before any more ignition parts, pressure test the cooling system, borescope that cylinder, and give me compression or leak-down numbers for every cylinder in writing."

Route 5 · Neither the cylinder nor its parts — crank signal, cold soak and software

The tell. The engine runs perfectly. There is no coolant loss, no smoke, no smell, and nothing changes when parts are swapped. The code appears only on cold mornings, or appears with a companion cylinder, or simply returns after every repair.

What it actually is: the measurement, not the cylinder. The computer infers a misfire from tiny changes in crankshaft speed, so anything that disturbs that signal produces misfire codes on cylinders that are firing perfectly. Lexus L-SB-0008-21 Rev1 is the clearest published case: on 2018-2021 RX 350 models it describes "companion cylinder misfire Diagnostic Trouble Code (DTC) combinations" — "P030100 and P030400 (Cylinders 1 and 4 Misfires Detected)", and the same for 2 with 5 and 3 with 6. The repair is a new crank angle sensor plate and an ECM reprogram, 7.2 or 7.9 hours, covered for "72 months or 70,000 miles." It adds a line worth quoting at a counter: "The ECM (PCM) should NOT be replaced as part of the Repair Procedure."

The cheap cause that gets skipped. The search. Subaru's 11-116-12R announces reprogramming files for P0301 through P0304 and says they are "intended to address a condition when DTCs are present for misfire, but no actual drivability condition exists or has been reported by the customer" — a manufacturer conceding its own threshold was too sensitive. Nissan's NTB22-001, for the 2022 Frontier, is keyed to codes that "only store during a cold start" and pairs an ECM reprogram with an intake manifold replacement. Mazda's 01-011-16-3139A covers a rough idle and misfire codes on 2013-2016 models when the car has been "parked overnight in cold conditions (below 32 degrees Fahrenheit / 0 degrees Celsius)." None of these is findable by staring at the code.

Where this does not apply. If coolant is dropping, or a plug came out oily, this is not your route. A calibration will not seal a cylinder.

At the shop — say this: "Before any more parts, search this VIN and engine for calibration updates and bulletins naming P0301 or a companion-cylinder misfire. If one applies, do it first and re-check the misfire counters."

What 315 federal complaints naming P0301 show

Our sample: every vehicle complaint in NHTSA's public files received 1 January 2015 to 8 September 2026, de-duplicated by report number — 777,040 unique owner reports. Narratives are ALL CAPITALS through 2020 and mixed case after, so every query is case-insensitive.

The set. Matching the code as a whole word returns 315 complaints, the same rule and the same count the Check Engine Code Index publishes. A looser match returns 336; we read all 21 extra rows and 19 are genuine reports whose code carries a scan-tool suffix or sits inside a range, so every count here is conservative by about six percent. Thirty reports drawn at random (seed 20260919) were read in full first: 0 of 30 false positives.

It is a money code, not a crash code. Every structured safety flag runs below the all-complaint baseline: towed 2.54 percent against 5.11, crash 0.95 against 4.37, fire 0.95 against 1.82, with two injuries and no deaths. But 22.9 percent describe a stall or a loss of power, 12.4 percent a flashing or blinking light, and 26.3 percent end with an engine, short block or long block replaced.

Where it lands on the clock. Only 84 reports state a usable odometer: median 80,320, quartiles 63,250 and 110,000, with 10 at or under 36,000 miles and 26 at or above 100,000. Model year 2017 accounts for 86 of the 315 and 2018 for 56 — the Ford EcoBoost cohort reaching the mileage where coolant intrusion shows itself. Reports per year: 32 in 2023, 62 in 2024, 57 in 2025 and 71 in 2026 counting only to 8 September.

The two medians on this page, reconciled. The index box at the top says 80,641 miles across 84 reports; this section says 80,320. Same 84 reports: the two middle values are 80,000 and 80,641, the index quotes the upper one and this analysis averages the pair, as a median of an even-numbered set conventionally is. The gap is 321 miles and changes nothing — but a reader comparing two Pulscar pages deserves to know which convention produced which number.

The finding that should change what you buy. 85 of the 315 reports (27.0 percent) say spark plugs or a coil were fitted, and 54 of those 85 — 63.5 percent — also say the fault came back. Read that with its caveat attached: this is a complaint file, and an owner whose $40 plug fixed the car never writes to a federal agency. The number does not mean ignition parts fail two thirds of the time. It means that among people angry enough to file, ignition parts were very often the money spent before the diagnosis. Against that, only 6 of 315 describe swapping a coil or part between cylinders, and only 20 of 315 mention a compression or leak-down test.

What 100 complaints say the fault turned out to be

Keyword counts tell you how often a word appears, not what the car had. So we drew a random sample of 100 of the 315 (Python's random.sample, seed 20260919), read every narrative to its end, and gave each a single primary outcome: what the report says a shop found or did, not what the owner suspects.

Read the first and last lines together. Coolant or a head gasket is the single largest outcome at 29 of 100. Ignition — the thing almost everyone buys first — is the reported action in 5. Add rings, compression and valvetrain to the coolant line and 53 of 100 are a sealing or mechanical problem in one cylinder.

The second-largest outcome is no outcome. Nineteen of 100 end with the owner still holding the code: not repaired because of cost, or a shop that could not reproduce it, or a diagnosis that never arrived.

The honest limits. One reader, one pass, no second coder, and a 100-row sample puts roughly nine percentage points of uncertainty around a 29 percent share. A borderline rule was written down and applied: a narrative that reports ignition parts fitted and a later, deeper finding is coded to the deeper finding, because the question is what the fault turned out to be. Both the rule and the per-report coding are in the published file.

When a single-cylinder misfire is a documented engine defect

Most P0301 stories end at a part. A small, important group ends at an engine the manufacturer has already written about, and in those cases the cylinder number is part of the evidence rather than a distraction.

Honda's piston rings, and a class action. Bulletin 13-081 and its sisters 13-078, 13-079, 13-080 and 13-082 exist because of the settlement of Soto et al. v. American Honda Motor Co., Inc., 3:12-cv-1377-SI (N.D. Cal.). The mechanism is unusually specific: the rings on certain cylinders rotate until their gaps align, oil passes into the chamber, that cylinder's plug fouls, and the computer reports a misfire on that cylinder alone. Honda's flat-rate note records that its 8.5-hour procedure is deliberately shorter than the service manual's because it "does not require the complete removal of the power steering pump, catalytic converter(s), engine and transmission, or the crankshaft."

Ford's coolant intrusion. The 1.5L and 2.0L EcoBoost family is the largest cluster in our file, and Ford's bulletins list P0301 among the codes and direct a short or long block replacement once a pressure test and a borescope confirm coolant in a cylinder. One owner recorded the physical signature: "the top of the cylinder 1 piston appears to be 'washed' which happens when fuel is not ignited during the combustion process." Our P0300 guide carries the documents and their clocks.

GM's valvetrain. 23-NA-043 replaces valve lifters and guides on 2020-2021 5.3L and 6.2L trucks and SUVs for misfire codes including P0301, and GM's older preliminary information documents for the same code family send technicians to plugs fouled with oil rather than to ignition faults.

Toyota's lash adjusters. T-SB-0010-24 is the newest of these and the most surprising: a cold-morning misfire on a Camry, Corolla, RAV4, Corolla Cross or Avalon, in a narrow coolant-temperature window, repaired by replacing all eight exhaust-side valve lash adjusters under the powertrain warranty.

What they have in common. In every one, the cheapest correct first step is a look, a test or a VIN check — never a part. And in every one, an owner who bought a coil first still paid for the real repair afterwards.

What 480 manufacturer documents naming P0301 look like

The Check Engine Code Index counts 276 distinct bulletins naming P0301 from 12 manufacturer groups, with software as the remedy in at least 41 percent overall but only 17 percent once Stellantis is set aside, and 7 of them mentioning a special coverage, warranty extension or emissions campaign — the highest coverage count of any code in the index. Those are the figures to quote.

We also ran the raw search, which applies none of the index's exclusions: 480 distinct NHTSA document IDs from 14 groups — Stellantis 172, Honda and Acura 117, Ford and Lincoln 67, VW Group 23, Toyota and Lexus 22, Mazda 21, Jaguar Land Rover 18, GM 11, Kia 9, and six each for Subaru, Hyundai and Nissan. The gap is the index removing helpline lists, Honda and Acura dealer messages, IDs with no digits and heavy-truck makers, and folding revisions into one bulletin.

The useful pattern is not the count. It is that the three largest publishers point in different directions: Ford at coolant and blocks, Honda at rings, plugs and injectors, Stellantis at multi-code software flashes naming P0301 among dozens of others. A bulletin search is only as good as the VIN and engine you search it with.

Which P0301 repairs does a manufacturer pay for, and which are yours? The documents draw that line, and the dates on them matter more than the badge. Honda's settlement bulletins cover misfire repairs that trigger P0301 through P0304 for 8 years with unlimited mileage from the original purchase date on named Accord, Odyssey, Pilot and Crosstour years. Toyota repairs cold-temperature misfire codes on Camry, Corolla and RAV4 under a 60-month, 60,000-mile powertrain warranty, and Lexus covers the RX 350 companion-cylinder repair for 72 months or 70,000 miles. Ford's coolant-intrusion programs carry their own terms. Against all that, a worn plug at 90,000 miles, a coil that finally gave up, and an engine damaged by months of driving on a flashing light are yours. Pulscar's free check gives a first read on the idle before the conversation about who pays begins.

What owners were quoted when it went past a part

The ladder above carries published estimate ranges. This is different: it is what owners told the federal government about the top of that ladder.

The method, briefly. All 315 narratives were split into sentences, and every dollar figure between $300 and $40,000 in a sentence naming an engine, block, head gasket, cylinder head, piston or ring was pulled out. Each was then read; diagnostic fees, unrelated repairs and vehicle values were dropped by reading rather than by rule.

The result: 30 figures. Median $8,960, quartiles $7,000 and $10,000, range $4,000 to $33,000, with 21 of the 30 at or above $8,000. Nineteen of the 30 are Ford, Lincoln or Mercury. The single largest is a 2019 Jaguar F-Type quoted $33,000; strip that one outlier out and the median barely moves, to $8,820.

Read it as shape, not as a price list. Thirty reports is not a market survey and none are invoices we have seen. Our P0300 guide published a separate set of 29 owner figures with a median of $9,000 — a different query on a different code, and the two landing within $40 of each other is a coincidence, not a measurement.

What the distribution actually shows. When a misfire goes past ignition and past injectors, it does not stop politely at $1,500. Several owners say so in the same sentence as the number: "almost double the value of the car if the engine was working", "surpassing the value of the vehicle three times over."

The three tests that settle it

Everything on this page reduces to three questions, and each has a test that produces an answer rather than an opinion.

1. Is it spark? The swap test. Move the coil, drive, watch the counters. Free, five minutes on most engines, and it either convicts or clears the most commonly replaced part in the misfire world. Repeat with the plug.

2. Is it fuel? The balance test. An injector balance or contribution test, plus fuel trims for that bank. It tells you how much that one cylinder is contributing, and whether the mixture is rich or lean before the injector is condemned. On accessible engines, injectors swap between cylinders exactly like coils.

3. Is it compression? The gauge and the leak-down. $100 to $300, and the only one of the three that can rule out the expensive branch. A cylinder that will not hold pressure will not be fixed by anything bolted to the outside of it. A leak-down test tells you where the pressure goes: hissing at the intake is an intake valve, at the exhaust an exhaust valve, into the crankcase the rings, bubbles in the coolant a head gasket.

Run them in that order and stop at the first answer. Twenty of our 315 complaints mention a compression test. Most of the rest describe buying parts.

Which of the three tests should you run first, and does the order matter? It matters more than which tools you own. Run the swap test first, because it is free and takes five minutes: if the misfire follows the coil to another cylinder you are finished, and if it does not you have cleared the most commonly replaced part in the misfire world. Run the fuel test second, because injector balance figures come off a scan tool the shop already has once the car is connected. Run compression last, because it is the only one that costs real money and the only one that can condemn an engine, and because the first two often make it unnecessary. Owners in the federal file mostly did the opposite: 85 of 315 reports describe ignition parts being fitted, and only 20 describe a compression test at all. Pulscar's free check adds a read on the idle while you work through them.

Should you buy a used car with a P0301 history?

This is the adjacent question the data answers best, because a single-cylinder misfire is one of the few check-engine faults you can test for in a parking lot.

Ask for a scan in front of you and look at three things. The codes, current, pending and history. The readiness monitors: if they read "not ready", the codes were cleared recently and the car's history has been erased. And the per-cylinder misfire counters after a cold start, the closest thing to a live confession an engine offers.

Then check the two cheap physical facts. Look at the coolant level and the reservoir, and pull one plug if the seller allows it. On our data those two checks separate a $200 problem from an $8,960 one more reliably than any other pair.

And run the VIN for coverage, not just recalls. Most documents on this page are warranty extensions and settlements: they follow the vehicle rather than the owner, but they expire on a date. A 2012 Odyssey with the ring repair already done under 13-081 is a different car from one that never had it and is now past 8 years from purchase.

Price it honestly. A car that passes all of the above deserves a small discount for the unknown. A car with a stored P0301, a low coolant level and a seller who "just topped it up" deserves the engine figure, not the coil figure. Warranty extensions owners do not know about covers how to check coverage that travels with the vehicle.

Clearing P0301 before an inspection: why it comes back

The light is an automatic fail. Under 40 CFR 85.2222, the federal procedure for OBD inspections, a vehicle whose check engine light is commanded on "shall fail the OBD inspection."

A freshly cleared car is not ready. The same rule allows 2001 and newer vehicles no more than one unset readiness monitor, and clearing resets them all. A misfire from coolant, rings or a lash adjuster returns on the next cold start anyway.

And the code is your evidence. Several of the coverages on this page are keyed to the fault being present, and a dealer who finds no code has nothing to claim against. One owner in our file was refused warranty work for exactly that reason, after a previous shop had cleared the codes to make the car drivable.

What actually happens at the shop, checkpoint by checkpoint

Checkpoint 1 — the VIN question. "What recalls, emission recalls, warranty extensions, special coverages and customer satisfaction programs show on this VIN, open or completed?" Free, and on a Honda V6 or a Ford EcoBoost it decides the bill.

Checkpoint 2 — the swap test and the plug. "Did the misfire follow the coil when you moved it, and what did the plug from that cylinder look like compared with its neighbours?"

Checkpoint 3 — the coolant and the numbers. "What did the cooling system pressure test and the borescope show, and what are the compression or leak-down figures per cylinder?" Get them written on the repair order.

Checkpoint 4 — the bulletin search. "Is there a calibration update or bulletin for this code on this VIN and engine, and has its procedure been done before any parts?"

Checkpoint 5 — the ceiling. "What is the diagnostic charge, and will you call me before anything above this figure?" The overcharging guide covers how quotes grow.

Three diagnostic traps that cost owners the most

Trap 1: the coil bought instead of tested. The situation: a lone P0301, a smooth-ish idle, 90,000 miles. What the shop says: "Coil on one. Three hundred and twenty dollars fitted." What is really happening: nobody moved the coil, and moving it is free. In our file, 85 reports mention ignition parts being fitted and 54 of those say the fault came back. Price versus bill: $0 for the test, against $150 to $400 for a part that may be innocent. Your protective question: "Did you swap the coil to another cylinder, and did the misfire follow it?"

Trap 2: ignition parts on a coolant-eating EcoBoost. The situation: a 2017 Escape or Edge, P0301 with P0316, coolant topped up twice this month. What the shop says: "Misfire on one. Plugs and a coil and you're good." What is really happening: Ford's bulletins for that engine list P0301 beside P0316, P0217, P1285 and P1299 and send the technician to a pressure test, a borescope and a block; in our Ford group, coolant or a head gasket is named in 84.7 percent of reports. Price versus bill: $300 to $600 of ignition parts before a $150 test that decides the real repair. Your protective question: "Did you pressure test the cooling system and look inside that cylinder with a borescope?"

Trap 3: the engine condemned before the extension was checked. The situation: a 2012 Honda Odyssey, one fouled plug, P0301, and a quote to rebuild. What the shop says: "It needs rings. It's an engine job." What is really happening: it may well need rings — and Honda's 13-081 covers exactly that repair for 8 years with unlimited mileage from the original purchase date, with a two-question flowchart to prove eligibility. Price versus bill: $0 inside the term, against a four-figure rebuild outside it. Your protective question: "Has this VIN been checked against bulletin 13-081, and is the plug from the affected cylinder fouled?"

Two quotes, decoded

These are composite scenarios, not individual customers — the mileages and outcomes are typical of the patterns in the documents and complaints above, assembled so the arithmetic is visible. We do not publish invented testimonials.

Scenario 1 — the coil that was innocent. A 2018 Escape at 74,000 miles, P0301 and P0316, a slightly rough idle and a coolant reservoir that has needed topping up twice. The first quote is plugs and a coil at $420. The owner asks for the swap test instead: the misfire stays on cylinder 1. A $150 cooling system pressure test and a borescope find coolant in the bore, and the conversation moves from a coil to a block — three weeks earlier, and $420 richer, than it otherwise would have. Lesson: the free test is not a shortcut past the diagnosis. It is the diagnosis starting properly.

Scenario 2 — the Odyssey inside the clock. A 2012 Odyssey bought new, 118,000 miles, P0301 and one heavily fouled plug. An independent shop quotes a rebuild. A Honda dealer runs the VIN against 13-081, confirms the plug on cylinder 1 is fouled, and the pistons are cleaned and the rings and plugs replaced under the settlement's 8-year, unlimited-mileage term. A car sold six months later would have been outside it. Lesson: with these codes, the purchase date is worth more than the odometer.

After the fix: verify, then prevent

Verify the counters, not the dashboard. Most readers show per-cylinder misfire counts. After a week that includes at least one cold start, they should read zero on every cylinder, and the code should not be back as pending.

Keep an invoice that names the cause. It should say what was tested, what the compression or pressure test showed, and which part was replaced and why — the sentence a future warranty claim or buyer will read.

Protect the converter and the engine. Treat a flashing light as a stop-soon event, fix oil or coolant consumption before it becomes a second repair, and keep the coolant level marked for a month after any sealing job. The check engine light guide covers the next light; the recording guide turns today's idle into a baseline.

Your action plan: 10 minutes, today, this week

Next 10 minutes — free:

1. Read and save every code, pending and history, plus the freeze frame and the per-cylinder misfire counters. Do not clear anything.
2. Check whether the light is flashing, and mark the cold coolant level with tape.
3. Note when the misfire happens: cold start only, under load, after a fill-up, or all the time.

Today:

4. Do the coil swap test and pull the plug from the named cylinder.
5. Run the VIN for recalls, then call a dealer and ask for warranty extensions and campaigns by name.
6. Run a free sound check on the idle before anyone quotes a part.

This week:

7. Ask, in writing, for a bulletin and calibration search on this VIN and engine.
8. If coolant is low or the plug looked wrong, ask for a pressure test, a borescope and compression or leak-down figures before any quote.
9. After the repair, drive through a cold start and confirm the misfire counters are at zero.

Sources and documents

Primary documents, downloaded from NHTSA's server and read as text. Model years, terms, part numbers, labor times and quoted phrases come from the documents themselves. ⚪ marks estimates and owner reports.

1. Honda Service Bulletin 13-081, version 19, 8 October 2020 — 2011-2013 Odyssey; "the piston rings on certain cylinders may rotate and align, which can lead to spark plug fouling" setting P0301 through P0304; powertrain warranty extended "to 8 years with unlimited mileage from the original date of purchase" under the settlement of Soto et al. v. American Honda Motor Co., Inc., 3:12-cv-1377-SI (N.D. Cal.); 8.5 flat-rate hours; salvage and branded titles excluded. Bulletin (PDF)
2. Honda SB 13-078, 13-079, 13-080 and 13-082 — the same settlement extension for the 2008-2012 Accord, 2010-2012 Crosstour, 2008-2010 Odyssey and 2009-2013 Pilot, read in NHTSA's bulletin flat file. Field layout
3. Lexus L-SB-0008-21 Rev1, 15 April 2021 — 2018-2021 RX 350; "companion cylinder misfire Diagnostic Trouble Code (DTC) combinations" P030100 with P030400; crank angle sensor plate 19315-31050 and ECM reprogram, 7.2 or 7.9 hours; "This repair is covered under the Lexus Powertrain Warranty ... 72 months or 70,000 miles"; "The ECM (PCM) should NOT be replaced as part of the Repair Procedure." Bulletin (PDF)
4. Toyota T-SB-0010-24 Rev2, 21 February 2024, revised 25 June 2025 — 2018-2024 Camry, 2019-2025 Corolla and RAV4, 2022-2025 Corolla Cross, 2021-2022 Avalon with A25A-FKS and M20A-FKS engines; misfire codes including P030100 "upon cold soak start up with engine coolant temperatures between 14°F – 41°F"; replace the exhaust-side valve lash adjusters, 2.8 to 3.0 hours, Toyota Powertrain Warranty 60 months or 60,000 miles. Bulletin (PDF)
5. Nissan NTB22-001, 12 January 2022 — 2022 Frontier; "DTC P0300 and/or P0301(2/3/4/5/6) is stored during a cold start"; reprogram the ECM and replace the intake manifold. Bulletin (PDF)
6. Lexus L-SB-0007-21 Rev2, 7 September 2024 — 2015-2024 GS, IS, NX and RC, 8AR-FTS engine, cylinder misfire DTCs. Bulletin (PDF)
7. Mazda 01-014-16-3208A (ignition coil internal failure on a large cold-to-warm ambient swing, 2013-2016 Mazda3, Mazda6 and CX-5), 01-011-16-3139A (rough idle and misfire codes after an overnight park below 32°F), 01-017-16-3242 (injector circuit codes P0201 to P0204, with misfire codes "usually stored at the same time") and 01-001-23a (turbo engines only) — read in NHTSA's manufacturer communication flat file.
8. Subaru 11-116-12R, 18 November 2016 — ECM reprogramming files for P0301 to P0304 "intended to address a condition when DTCs are present for misfire, but no actual drivability condition exists or has been reported by the customer." Read in the flat file.
9. GM 23-NA-043 — 2020-2021 Silverado, Sierra, Suburban and Escalade 5.3L and 6.2L; replace the valve lifter and guides for misfire codes P0300 through P0308. PIP4525D — 2008-2009 full-size trucks and SUVs, misfire with oil consumption, P0300 and/or P0301, "inspect all of the spark plugs for oil." PIP3333C and PIE0505 — intermittent misfire and a field data request on 2018-2019 Equinox and Terrain. Read in the flat file.
10. Ford coolant-intrusion bulletins for the 1.5L and 2.0L EcoBoost, which list P0301 beside P0300, P0316, P0217, P1285 and P1299 and direct short or long block replacement — documented in full, with their programs and clocks, in our P0300 guide.
11. 40 CFR 85.2222 — OBD inspection and readiness monitor rules. Regulation
12. Pulscar Check Engine Code Index — 276 bulletins naming P0301 from 12 groups; software in at least 41 percent overall and 17 percent without Stellantis; 7 coverage or campaign mentions; 315 complaints; median 80,641 miles. Index · Code table (CSV) · Index method
13. Our P0301 analysis — 315 complaints matched on the whole word, 100 of them read in full and coded; the cylinder-code comparison; 480 manufacturer documents by group; 30 owner-reported engine figures. Method · by make (CSV) · top models (CSV) · what owners name (CSV) · cylinder codes compared (CSV) · the 100 read (CSV) · owner engine figures (CSV) · bulletins by group (CSV) · CC BY 4.0
14. Our complaint sample — 777,040 unique vehicle complaints received 1 January 2015 to 8 September 2026. Complaint field layout

What we looked for and did not find. No recall campaign in the federal file is keyed to a single cylinder; the misfire recall picture — 27 campaigns in sixteen years, three of them light passenger vehicles — was measured for our P0300 guide and is cited rather than recomputed. Several GM and Lexus preliminary information documents are indexed by NHTSA without a retrievable PDF at the static paths we tried; those are quoted from the flat-file summary and labelled as such. And we found nothing supporting the common claim that cylinder 1 fails more often than its neighbours.

Related: P0300 random misfire, the code index, check engine light, flashing light, P0420, P0171, spark plugs, fuel injectors, head gasket, white smoke, engine knocking, rough running, diagnostic costs, warranty extensions, our story.

Methodology. Model years, terms, part numbers, labor times and quoted phrases come from the documents above, downloaded from NHTSA's server and read as text; a few preliminary information documents were read from NHTSA's flat-file summaries because no PDF was retrievable. Complaint counts come from NHTSA's public complaint flat files, de-duplicated by report number; they are owner reports, not audited repairs, and they are not failure rates. Price ranges are estimates carried forward from our P0300 and parts cost guides and marked ⚪. Coverage depends on your VIN and your purchase date.

Prices verified September 2026 and reviewed quarterly — no estimator site would serve our checks in this pass, so the shop-estimate ranges are carried forward from our P0300 guide unchanged and remain marked as estimates; the 30 owner-reported engine figures were extracted and read this month. Technical documents verified September 2026: every bulletin, regulation and index figure cited above was opened and read on 19 September 2026.

Had a P0301 found and fixed, or paid for a coil that did not fix it? Email [email protected] with the year, engine, mileage and what the swap or compression test showed — owner data is how these ranges get sharper.