P0304 Code in 2026: Cylinder 4 Misfire, and What Number 4 Means on Your Engine

P0304 is one code with several different meanings, and which one you have depends on what is under your hood.

The engine computer watches the crankshaft speed up with each cylinder's power stroke. When cylinder 4's stroke is weak often enough, it counts misfires and stores P0304 — "Cylinder 4 Misfire Detected." The code names a place, not a part. On an ordinary four-cylinder, cylinder 4 is just one of four, and the usual suspects apply. But on a Honda V6, cylinder 4 is one of the four cylinders its cylinder management system switches off. On a Chrysler 3.6L, it sits on the left head that Chrysler extended the warranty on. On a GM V8 with Active Fuel Management, it is one of the four cylinders whose lifters fold. On a Mazda 2.5L with cylinder deactivation, it is one of two that stop firing on the highway. And on a 2017-2021 Civic Type R, it is the plug hole that collects water from the hood scoop.

We searched NHTSA's complaint file — 780,071 complaints received from 2015 to September 2026 — and found 345 complaints that name P0304. Then we read a random 50 in full. Fifteen mentioned P0304 only inside a code range copied from a bulletin; in the 35 where the car really stored it, 22 ended inside the engine — coolant, rings, valves or a failed engine — and one ended at an ignition part. Two manufacturers dominate the file, with opposite stories. 127 are Fords and Lincolns, and 108 of them mention coolant. 95 are Hondas and Acuras, and they mention injectors, piston rings and warranty extensions, almost never coolant.

By the end you will know what cylinder 4 is on your engine, which of five routes your P0304 is on, what each costs in 2026, which extension or bulletin applies, and the cheap cause that gets skipped when a coil is quoted first. Our P0301 guide covers the free coil swap test and the misfire family in depth; this one is about what makes cylinder 4 different.

Claims marked 🟢 rest on a recall, extension, bulletin or program listed in the Sources section; 23 of those documents were downloaded and read in full. Claims marked ⚪ are shop-price estimates carried from our repair guides. Owner-reported figures are what people told NHTSA, published with ODI numbers. The complaint set is a searched set: counts are pattern flags checked against the matched text, and the complaints quoted or counted by hand were read in full.

I built PulsCar after a $380 "full diagnostic" in Phuket in 2025 led to roughly $6,000 over six months across several shops (the story is on the About page). A misfire code is where that kind of spiral usually starts — a coil, then plugs, then injectors — so every claim below that rests on a document names it.

What number 4 means on your engine

Where cylinder 4 physically sits — which end, which bank — comes from the service information for your exact engine, not from a forum diagram. Our P0301 guide explains why bank 1 is often the rear bank on a transverse V6 and why owners work on the wrong side.

Why this matters before you spend anything. On a Honda V6, a P0304 that sits beside P0301, P0302 or P0303 is the pattern Honda's piston ring documents describe. On a Pentastar, P0304 with P0302 or P0306 is the left head. On a GM V8, P0304 with a tick is a lifter until proven otherwise. None of those is fixed by a coil. On a four-cylinder with nothing special about cylinder 4, the coil swap test is still the best first ten minutes.

Is cylinder 4 special? What 1,027 single-cylinder complaints show

A whole-word search on P0304 overcounts, because owners paste bulletin text like "P0301-P0304." So to ask which cylinder each engine actually names, we used only complaints naming exactly one cylinder code and not P0300: 1,027 complaints.

Across all of them, cylinder 4 is not the leader. Cylinder 2 is named alone in 305, cylinder 3 in 259, cylinder 1 in 191 and cylinder 4 in 184. But by model the picture splits:

Three things stand out. On the Silverado 1500 — a truck sold mostly with V8s — cylinder 4 is named in 8 of 21 — two of them the same report filed twice, one a 2.7L four-cylinder — still more than one cylinder in eight would be; one confirms a failed lifter, and on the V8s cylinder 4 is one of GM's deactivating cylinders. On the Jeep Wrangler, cylinders 2 and 4 together account for 15 of 19 — both left-bank cylinders on the 3.6L. And on Honda's V6 vans and SUVs, the Odyssey's 32 single-cylinder complaints are all on cylinders 1 to 4, none on 5 or 6, and the Pilot's are 26 of 30 — the cylinder group Honda's own documents single out.

On a Ford EcoBoost, cylinder 4 is the least-named of the four — 27 of 199 on the Escape against 82 for cylinder 2. The fault behind them is the same coolant intrusion; the number is just where it showed up. The by-model file is published: every model with five or more complaints.

These are complaint counts, not failure rates, and small models move with a handful of reports. Use them to decide what to test first, not to rank engines.

First: is this really P0304? The codes around it

What matters most here: read every stored code, not just the first. The companion codes decide whether the next step is a ten-minute coil swap or a pressure test.

The 15-minute P0304 check

1. Read every code and the freeze frame. Note whether it set cold or hot, at idle or under load.
2. Look at the coolant reservoir, cold. Below the line with no visible leak, plus P0304, is the first thing to report to a shop.
3. Smell the exhaust at a cold start. Sweet, or white smoke that does not clear, points at coolant.
4. Find cylinder 4 in the service information for your engine.
5. Swap coil 4 with a neighbor, clear the codes and drive. If the misfire moves, it is the coil. If it stays on 4, it is not.
6. Pull the number 4 plug and look: oily, wet, white, or with a cracked insulator. On a Type R, look for water in the well.
7. Listen for a tick at idle. On a V8 or a Mazda with deactivation, a tick plus P0304 is valvetrain.
8. Run the VIN for recalls and warranty extensions on the manufacturer's site.
9. If the light is flashing, stop driving until the cause is known.

Eight P0304 situations, and which is yours

"A 2013-2017 Odyssey or 2013-2015 Pilot with a fouled plug on cylinder 4." Honda's ring extension. Route 2, and the Honda V6 section.

"A Ford Escape, Edge, Fusion or MKC with P0304 and falling coolant." Coolant intrusion. Route 4, and the Ford section.

"A Civic, Accord or CR-V with P0304 and coolant loss." Honda's August 2026 head gasket extension. Route 2.

"A Silverado, Sierra, Tahoe or Yukon with P0304 and a tick." A deactivating lifter. Route 5.

"A 2018-2019 CX-5 or Mazda6 with P0304 and lost power." A rocker in a deactivating cylinder. Route 5.

"A Jeep, Dodge or Chrysler 3.6L with P0304 and P0302 or P0306." The left head. Route 4.

"A Honda Fit, CR-V or Pilot where the dealer quotes injectors." Route 3, and check the injector and carbon extensions first.

"New plugs and coils, and P0304 is back." Route 4 or Route 5, and the section on why.

The P0304 price ladder, from the bottom up

Read it from the bottom up: most of the expensive rows are covered, for some cars, by an extension or program — Ford's 21N12, Honda's ring, injector and head gasket extensions. The three free steps at the top decide which row you are in.

How much does it cost to fix a P0304 code? Anywhere from nothing to an engine, and the first ten minutes decide which. Our repair guides carry $80 to $400 for spark plugs and $150 to $400 for a coil, and a coil swap test costs nothing. In PulsCar's reading of the federal complaint file, the big figures are mechanical: 17 Ford EcoBoost owners reported $4,700 to $11,000 for engines after coolant reached a cylinder, two Jeep 3.6L owners $3,700 and $4,850 for a cylinder head, and Honda V6 owners up to $12,000 for piston ring and short block work. Honda injector repairs ran $899 to $2,000. Several of those repairs sit inside warranty extensions — Ford's 21N12 and Honda's ring, injector and head gasket programs — so the VIN check comes before the quote.

The P0304 number, by what you drive

Full tables: 345 complaints, flags · recalls, extensions and bulletins.

The five P0304 routes, cheapest first

Route 1 · Spark on cylinder 4 — plug, coil, boot and a wet plug well — $0 to $1,100

The spark is where almost everyone starts, and it should be — not because it is the likely cause, but because it is the cheapest to rule out. A coil swap takes ten minutes on most coil-on-plug engines. If the misfire follows the coil to cylinder 3, you have your answer for $150-$400 ⚪. If it stays on 4, you have saved that money.

What the documents show. The most cylinder-4-specific spark fault in the file is Honda's. In July 2023, 🟢 Honda asked dealers for 2017-2021 Civic Type Rs that "Must only have DTC P0300 and/or P0304 stored," adding that "If P0301, P0302 or P0303 are stored, the vehicle does not qualify." Two years later 🟢 bulletin 25-062 named the cause: "Water could enter the hood scoop and accumulate down inside the no. 4 spark plug hole. This can corrode the ignition coil, plug tube, and spark plug, leading to a misfire to the no. 4 cylinder." Honda redesigned the cylinder head cover so that it "will divert water away from the no. 4 spark plug hole and ignition coil." The repair is 1.4 hours for a new cover, plug and coil — or 13.6 hours for a cylinder head when the plug tube is badly corroded, with photos required at 12 mm of corrosion or more — under the normal warranty. The same kind of fault, water pooling in plug wells, is behind Ford's Ranger bulletin in our P0300 guide, and Kia's and Hyundai's all-four-coil and all-four-plug campaigns are there too.

What the owners report. A 2014 Altima owner was quoted $357 for one coil and $1,023 for all four coils and plugs. A 2018 Fit owner paid $885 for coils and a catalytic converter and had new codes two days later. A 2021 CR-V Hybrid's dealer swapped plugs and coils, drove it 163 miles, and it went into limp mode on the highway six days later. The spark was replaced far more often than it was the answer.

The cheap cause that gets skipped. The plug well itself. Pull the coil and look down the hole with a flashlight before buying anything: water, rust or oil in the well is a finding, and on a Type R it is Honda's documented cause.

Fix it yourself — the honest split. A coil swap, a look at the plug and the well, and reading the misfire counters with a $20-$40 reader are driveway jobs on most four-cylinders. Rear-bank plugs on a transverse V6 often mean removing the intake plenum; pay for that.

At the shop — say this: "Before replacing parts, can you swap the cylinder 4 coil with a neighbor and see if the misfire follows it, and show me the cylinder 4 plug and plug well?"

Route 2 · Your VIN is on an extension or program — $0

More P0304 repairs are covered by warranty extensions than by recalls, and more of them belong to Honda than to anyone else.

What the documents show. 🟢 Honda's 26-001 and 26-002, February 2026, extend coverage for misfire "that triggers DTCs P0301 thru P0304" on 2013-2017 Odysseys and 2013-2015 Pilots "to 10 years from the original date of purchase or 150,000 miles, whichever comes first." The cause in Honda's words: "The piston ring function deteriorates due to ring wear and sticking from sludge. Oil from the crankcase adheres to the spark plug." 🟢 Honda's March 2021 letter extends software and injector repairs on 2016-2017 Pilots, 2017 Ridgelines and 2018 Odysseys to the same 10 years or 150,000 miles. 🟢 2015-2016 Fits have a 10-year, 150,000-mile injector extension for injectors that "may not have been installed to proper specifications," and 🟢 2018-2019 Fits one for "valve cleaning and software installation due to carbon buildup." 🟢 On 26 August 2026 Honda extended head gasket coverage on "certain 2018-2026 Accord, 2018-2026 Accord Hybrid, 2016-2026 Civic, 2025-2026 Civic Hybrid, 2017-2026 CR-V and 2020-2026 CR-V Hybrid vehicles" for "overheating, coolant loss, misfire, and/or DTCs P0300 through P0304," to 10 years or 150,000 miles, "whichever comes first," with "a 6-month grace period from the date of customer notification" for cars already too old. 🟢 Ford's 21N12 provides "a no-cost, one-time repair (if needed) to the 1.5L short block, due to coolant intrusion to the cylinder bores, for 7 years of service or 84,000 miles from the warranty start date." And 🟢 Toyota's T-SB-0010-24 covers cold-start misfire on A25A and M20A engines under the 60-month, 60,000-mile powertrain warranty.

What the owners report. A 2013 Odyssey owner paid $2,686.36 at a dealer and then found the earlier ring extension; the VIN was not included. A 2015 Odyssey owner was quoted over $6,000 six weeks after the original warranty ended, received Honda's extension letter in 2025, and was refused again "because the codes were not present that day." A 2017 CR-V owner at 140,000 miles describes a Honda technician finding coolant reaching cylinder 4 through the head gasket, in a complaint filed in April 2026 — four months before Honda's head gasket extension.

The cheap cause that gets skipped. The date. Coverage windows open and close: Honda's 2025 ring window ignored age and mileage until 31 January 2026, and its successor keeps a 10-year, 150,000-mile limit. A repair already paid may be refundable: Honda's 2025 ring letter says owners who "previously paid" may be eligible "even if you no longer own the vehicle."

At the shop — say this: "Please run my VIN for warranty extensions and campaigns covering P0304 — piston rings, injectors, carbon, head gasket or coolant intrusion — and tell me in writing whether my car qualifies and what test decides it."

Route 3 · Fuel and air on cylinder 4 — injector, carbon and cold starts — $0 to $2,000

On a direct-injection engine the injector sits in the combustion chamber and the intake valves never see fuel to wash them. Both habits create cylinder-specific misfires: an injector that leaks or clogs, and carbon that builds on one cylinder's valves more than another's.

What the documents show. 🟢 Honda's 21-007 lists the possible causes for 2015-2020 Fit misfires in one line: "A faulty ignition coil, spark plug, fuel injector, or carbon buildup on the intake valves." 🟢 Honda's 2018-2019 Fit extension adds that "A combination of carbon deposits on the intake valves and the PGM-FI software settings may lead to reduced combustion stability," and a Honda service article, summarized in NHTSA's file, tells technicians on Fits in a stated VIN range to "start by doing service bulletin 21-007 (Carbon Buildup with Misfire DTCs) instead of replacing fuel injectors." 🟢 Toyota's T-SB-0010-24 is the cold-start case: misfire codes including P030400 "upon cold soak start up with engine coolant temperatures between 14°F – 41°F" on A25A and M20A engines, fixed not with injectors but with eight exhaust valve lash adjusters.

What the owners report. Twenty-eight of the 95 Honda and Acura complaints naming P0304 mention injectors. Nine Honda owners reported $899 to $2,000 for injector work — CR-Vs, Accords and Pilots, some with "4th fuel injector faulty," several quoted all four. A 2016 Pilot owner paid $1,311.19 for all injectors and rails at 46,473 miles — filed in 2020, a year before Honda's V6 injector extension for that model year, which runs 10 years from purchase. And a 2018 Camry owner's shop moved the cylinder 4 injector to another cylinder and "the same error keeps coming back cylinder 4" — on cold starts only, exactly the pattern of Toyota's bulletin, which points away from the injector.

The cheap cause that gets skipped. Swapping the injector. Like the coil, an injector can be moved to another cylinder on many engines; if the misfire does not follow it, the injector is innocent — and a cold-start-only misfire on a Toyota A25A points at the bulletin, not at fuel.

Fix it yourself — the honest split. Reading misfire counters and noting cold versus hot is yours. Injector swaps on direct-injection engines involve high-pressure lines and new seals every time; that is a shop job.

At the shop — say this: "If you suspect an injector, can you show me the test that singles out cylinder 4 — a swap, a balance test or a flow test — and check my VIN for Honda's injector or carbon extensions before quoting all of them?"

Route 4 · Cylinder 4 is not sealing — coolant, rings and a cracked head — $1,000 to $12,000

This is where most P0304 money goes. In our random 50, 10 of the 35 cars that really stored P0304 ended at coolant or a head gasket, and four at rings or oil consumption.

What the documents show. 🟢 Ford's 2.0L bulletin 19-2172 and its successors, for 2015-2018 Edge and 2015-2019 Escape, Fusion, MKC and MKZ, list "P0300, P0301-P0304, P0316, P0217, P1285 and/or P1299" and say "This may be due to coolant intrusion into the cylinder. To correct the condition, follow the Service Procedure steps to replace the long block engine assembly." 🟢 Ford's 20-2100 does the same for the 1.5L with "the short block and head gasket." 🟢 Honda's ring documents describe the other mechanism — oil past the rings onto the plug. 🟢 Chrysler's 09-002-14 Rev B, the X56 extension for 3.6L engines in nine 2011-2013 models, tells dealers to "Perform a cylinder leakage test on cylinders 2, 4 and 6" and, at "leakage of 25% or greater," to "Replace the left cylinder head."

What the owners report. A 2017 Escape's dealer swapped coils 3 and 4, the misfire stayed on 4, a plug swap seemed to clear it — and after the dealer let it sit overnight, "Dealer found coolant leaking into cylinder 4." Quote: $8,585.03. A 2019 Mustang 2.3L owner's dealer ran a borescope in cylinders 2 and 4 and found coolant: $10,728.85. A 2013 Wrangler owner was told a leak test showed "the 4TH CYLINDER HEAD WAS BAD" — $3,700, and not covered under X56. A 2015 Sportage owner with oil consumption was told cylinders 2 and 4 "had no compression."

The cheap cause that gets skipped. The long pressure test. Coolant intrusion on these engines can be invisible on a quick check: Ford's bulletins pressurize the cooling system, "hold for 5 hours," then use a borescope to look for coolant in the cylinders. It costs a diagnostic fee, not a coil.

At the shop — say this: "P0304 with coolant going down: can you pressure-test the cooling system, hold it for several hours, and borescope cylinder 4 — or do a leak-down — before quoting parts? And please check for a warranty extension on my VIN."

Route 5 · Valvetrain and cylinder deactivation — lifters, rockers and lash — $0 to $10,500

Cylinder deactivation saves fuel by shutting off some cylinders at light load. The shut-off cylinders have their own lifters, rockers and oil valves, and on several engines cylinder 4 is one of them.

What the documents show. 🟢 GM's PIP4568R covers V8s with AFM from 2005 to 2014 and describes "engine misfire on cylinder 1, 4, 6, or 7 and/or tick noise" caused by "an AFM lifter that unlocks as soon as the engine is started or one that is mechanically collapsed/stuck all of the time." It tells technicians to "inspect the cylinder 1, 4, 6, and 7 rockers and valves while cranking." 🟢 GM's PIP5584 lists "P349B Cylinder 4 Deactivation Performance" among the codes on 2019 Silverado and Sierra 5.3L and 6.2L engines, and replaces "the OCV for the cylinder with the concern." 🟢 GM's 23-NA-043 replaces valve lifters and guides on 2020-2021 5.3L and 6.2L trucks and SUVs built from 1 September 2020 to 31 March 2021, for misfire codes P0300 through P0308. 🟢 Mazda's SA-022/19 tells technicians facing P0304 with P0340, P3400, P349B or P3498 on 2018-2019 CX-5 and Mazda6 to "Inspect for fallen or broken rocker arm, especially in cylinder deactivation area (cylinders 1 and 4)" — and to call for "engine replacement authorization." 🟢 A Mazda service alert adds a cheap cause for P3400: the deactivation oil valve "may be plugged with plastic debris from the oil filter wrapper," which "may enter the center of the oil filter when poking a hole in the filter wrapper during removal." 🟢 Honda's 13-055 shows the V6 version: on 2013 Odysseys and Pilots, cold-start oil pressure could activate the cylinder management spool valves, "causing cylinders 1 through 4 to go into cylinder cut mode even though the ECU did not activate the VCM spool valve."

What the owners report. A 2020 Silverado owner was quoted $4,300 for "all lifters on effected side." A 2019 Yukon XL had zero compression on cylinders 4 and 6 with "Both lifters" collapsed — an engine at $10,479.57. A 2022 Silverado 2.7L's dealer found a "Cylinder 4 exhaust roller follower failure causing exhaust camshaft failure and bent exhaust valves" at $10,371.34. A 2018 CX-5 owner was told a rocker arm had "fallen off" and that, according to Mazda, the car was the 19th it had seen with the problem.

The cheap cause that gets skipped. Listening. A lifter or rocker announces itself with a tick before it fails, and a GM technician can run the AFM compression test in the bay. On a Mazda with P3400, the oil valve and its passage come before the head.

At the shop — say this: "There's a tick with P0304. Before plugs or coils, can you run the cylinder deactivation compression test or inspect the cylinder 4 rockers and lifters, and check the deactivation oil valve?"

The Honda V6 file: why its documents stop at P0304

Ninety-five of the 345 complaints are Hondas and Acuras, and the V6 vans and SUVs among them tell a story Honda's documents confirm in a way the number alone never would.

The pattern in the complaints. In the 32 Odyssey complaints that name a single cylinder, every one is cylinder 1, 2, 3 or 4 — none is 5 or 6. The Pilot's are 26 of 30. On an engine with six cylinders, that is not chance.

The pattern in the documents. 🟢 Honda's settlement bulletins from 2013 onward — 13-078 for the 2008-2012 Accord V6 and its sisters for other models — cover misfire "that triggers DTCs P0301 through P0304," for 8 years with unlimited mileage from the original purchase, because "The piston rings on certain cylinders may rotate and align which can lead to spark plug fouling." 🟢 The 2025 and 2026 extensions for the 2013-2017 Odyssey and 2013-2015 Pilot use the same code range. And 🟢 bulletin 13-055 explains which cylinders those are: the ones Honda's cylinder management can switch off — "cylinders 1 through 4."

What it means for you. On these engines P0304 is not a random cylinder. It belongs to the group whose rings, plugs and deactivation hardware Honda has written about for thirteen years, and a P0304 on a 2013-2017 Odyssey or 2013-2015 Pilot today is inside a 10-year, 150,000-mile extension if the car is inside those limits — in 2026, mostly 2016-2017 Odysseys bought new. Honda's corrective action starts with "update the PCM software, replace affected spark plugs then clean pistons and replace the piston rings," and moves to "engine block replacement" if the cylinder wall inspection fails. Its owner letter warns the job "may take up to 10.5 hours." Our Odyssey and Pilot guides carry the rest of those vehicles' records.

The honest limit. Complaints do not show which cylinder a healthy engine would misfire on first, and Honda does not say that cylinder 4 is worse than 1, 2 or 3. The finding is narrower and more useful: on these V6s, Honda's ring documents cover P0301 to P0304, and P0305 and P0306 are not in them.

The Ford EcoBoost file: the same fault, a different number

One hundred and twenty-seven complaints are Fords and Lincolns — Escape 55, Edge 33, Fusion 19, MKC 6 — and 108 of them mention coolant.

The documents. Ford's bulletins for the 1.5L and 2.0L EcoBoost do not single out cylinder 4. They list "P0300, P0301-P0304" together, because coolant can reach any cylinder, and send dealers to a pressure test, a borescope, and a short or long block. 🟢 Ford's 21N12 program pays for the 1.5L short block in certain 2017-2019 Escapes built to 8 April 2019 and Fusions built to 10 June 2019, for 7 years or 84,000 miles from the warranty start date; we found no equivalent program for the 2.0L.

What the complaints add. Where a single cylinder is named, cylinder 4 is the least common on these engines: 27 of 199 on the Escape, 20 of 139 on the Edge, 8 of 70 on the Fusion, against 82, 56 and 30 for cylinder 2. So P0304 on an EcoBoost is not a milder or different fault. It is the same coolant intrusion, found in a less common place. Owners reported $4,700 to $11,000 for engines, and at least two describe a dealer finding coolant in cylinder 4 specifically.

What to do with that. On a 2015-2019 1.5L or 2.0L Escape, Edge, Fusion, MKC or MKZ, P0304 with coolant loss is a stop-driving code. Ask for the pressure test and borescope, ask whether 21N12 applies, and read our P0300 guide and our Escape guide for the program history before you accept a quote.

The Chrysler 3.6L file: cylinder 4 is on the left head

On the Pentastar V6, the even-numbered cylinders — 2, 4 and 6 — share the left cylinder head, and that head is the one Chrysler wrote about.

🟢 What X56 says. Bulletin 09-002-14 Rev B, December 2014, covers 3.6L engines in the 2011-2013 Journey, 2012-2013 Wrangler, 2011-2013 200 and Avenger, Challenger, Charger, 300, Grand Caravan and Town and Country, Durango and Grand Cherokee when the computer stores "P0300- Multiple Cylinder Misfire," "P0302- Cylinder 2 Misfire," "P0304- Cylinder 4 Misfire" or "P0306- Cylinder 6 Misfire." The technician runs a leakage test on those three cylinders and replaces the left head at 25 percent leakage or more. It does not list P0301, P0303 or P0305.

What the complaints show. Single-cylinder complaints on the Grand Caravan name cylinder 2 in 18 of 25; on the Wrangler, cylinders 2 and 4 in 15 of 19. The owners' words match the bulletin's geography: a 2013 Grand Caravan with "P0306, P0304, P0302 due to left side head needing replacement"; a 2014 Town and Country whose dealer "CONFIRMED PISTON 4 ON LEFT SIDE" but whose model year falls outside X56; a 2015 Wrangler with "the well know Pentastar Tick" and P0304, where the dealer called for "new rockers and a cam shaft on the drivers side engine bank." A 2018 Wrangler at 42,609 miles needed "a new camshaft and lifters" for P0304, and the owner waited a month for parts.

What to do with that. On a 3.6L, P0304 alone or with P0302 or P0306 is a left-bank question, and a tick makes it a rocker and camshaft question. A leak-down test on 2, 4 and 6 is the manufacturer's own first step. The X56 years are 2011-2013 (2012-2013 for the Wrangler), and our Wrangler guide covers how that extension has aged.

The GM truck file: cylinder 4 is a deactivating cylinder

GM's V8s with Active Fuel Management or Dynamic Fuel Management shut off cylinders at light load, and cylinder 4 has been one of them for twenty years.

🟢 The documents, in order. PIP4568R, for 2005-2014 V8 trucks, SUVs and cars, describes "engine misfire on cylinder 1, 4, 6, or 7 and/or tick noise" from an AFM lifter that is "mechanically collapsed/stuck," and gives the test: "If the running compression of the misfiring cylinder stays below 25 PSI regardless of the AFM solenoid being commanded on or off," the lifter is at fault. 🟢 PIP5584 covers the 2019 redesign with a code for every cylinder, including "P349B Cylinder 4 Deactivation Performance." 🟢 23-NA-043 replaces lifters and guides on 2020-2021 5.3L and 6.2L engines built in a stated window. And on GM's 3.6L V6, 🟢 PIP5493, April 2017, told dealers that on a 2017 LaCrosse with "P0300 or P0304 misfire in history on Cylinder 4," a first visit should "clear code(s) and drive the vehicle," and "do not replace any parts at this time" — "Engineering is aware of this specific condition and is working on a solution."

What the complaints show. A 2019 Yukon XL with zero compression on cylinders 4 and 6 — both deactivating cylinders — and "Both lifters" collapsed. A 2020 Silverado with lifter failure on the side carrying cylinder 4. A 2023 Silverado 6.2L with P0304, a tick and a dealer note that the top-end noise had to be diagnosed before a recall could be performed. That recall is 🟢 25V-274, which covers connecting rods and crankshafts on 597,571 2021-2024 6.2L trucks and SUVs — a different part from the lifters, and one reason a dealer diagnoses the noise first.

What to do with that. On a GM V8, P0304 with a tick is a lifter question before it is a coil question. Ask for the AFM compression test, and on a 2021-2024 6.2L, ask separately whether 25V-274 is open. Our Silverado guide carries the lifter bulletins in full.

The Mazda file: cylinders 1 and 4, and a rocker on the floor

Mazda's 2.5L with cylinder deactivation, sold from 2018, runs on two cylinders at steady highway speeds. The two it shuts off are 1 and 4.

🟢 The alert. Service alert SA-041/18, December 2018, replaced by 🟢 SA-022/19 in May 2019, covers 2018-2019 CX-5 and Mazda6 with cylinder deactivation when the customer reports "lack of power, rough idle, engine noise" with P0304, P0340, P3400, P349B or P3498. The inspection is blunt: "Inspect for fallen or broken rocker arm, especially in cylinder deactivation area (cylinders 1 and 4)." The repair is a call to Mazda's hotline "for engine replacement authorization." Mazda later established a new cylinder head assembly as a service part for the deactivation engine, according to its bulletins in NHTSA's file.

What the owners report. A 2018 CX-5 owner describes limp mode while passing on a two-lane road, a misfire found on cylinder 4, and a rocker arm that had "FALLEN OFF" — and says that, according to Mazda, the car was the 19th it had seen, a count that reached 26 by the time the car left the shop. Another 2018 CX-5 owner reports P0304 on the highway several times, and cites a different Mazda bulletin, about condensation in the exhaust silencer, that we could not find in NHTSA's file.

The cheap cause Mazda documented. Before any head comes off for a deactivation code, 🟢 a Mazda alert covering 2016-2021 CX-3s and 2018-2021 Mazda6, CX-5, Mazda3 and CX-30 models says the deactivation oil valve can be "plugged with plastic debris from the oil filter wrapper," pushed in "when poking a hole in the filter wrapper during removal." Mazda adds that "Warranty does not apply if condition is created from outside influence" — worth knowing if a quick-lube shop changed the oil last.

When P0304 comes with P0301: companion cylinders

On a V6 whose firing order pairs them, cylinders 1 and 4 reach top dead center together, one on its power stroke and one on its exhaust stroke. Which cylinders pair depends on the engine: on a V6 that fires 1-4-2-5-3-6, cylinder 1 pairs with 5 instead. Anything that confuses the computer about crankshaft position — a damaged tone wheel, a sensor rotor with the wrong profile, a signal glitch — can land misfire counts on both at once, with nothing wrong in either cylinder.

🟢 Lexus's L-SB-0008-21 Rev1, 2021, is the clearest example in the file. It covers 2018-2021 RX 350s with "companion cylinder misfire" combinations — "P030100 and P030400 (Cylinders 1 and 4 Misfires Detected)," and likewise 2 with 5, and 3 with 6. The repair is not plugs, coils or injectors: it replaces the "crank sensor rotor" and reprograms the engine computer, under the Lexus powertrain warranty of "72 months or 70,000 miles."

The practical rule. On a V6 that pairs them, P0304 together with P0301 — and no fault in either cylinder when a coil or injector is swapped — points at the crankshaft signal, not at two cylinders failing at once. Our P0335 guide covers the crankshaft sensor and the wheels it reads.

What 345 complaints naming P0304 show

Our sample: every vehicle complaint in NHTSA's public files received 1 January 2015 to 21 September 2026, de-duplicated by report number — 780,071 unique owner reports. Every query is case-insensitive, and PO304, written with a letter O, is included.

The set. A whole-word search for P0304 returns 345 complaints. Of those, 184 name no other cylinder code and not P0300, and 61 contain a code range such as "P0301-P0304," usually pasted from a Ford or Honda bulletin — which is why we read a random 50 before counting anything as a cylinder 4 failure.

It is getting more common in the file, not less. Complaints naming P0304 ran 3 to 24 a year from 2015 to 2022, then 38 in 2023, 58 in 2024, 72 in 2025 and 79 in 2026 through 21 September — the 2017-2019 Fords and 2013-2017 Hondas reaching the mileage where their documented faults show.

What the flags show. 120 mention coolant or antifreeze (35 percent), 97 spark plugs, 82 an engine or block replaced, 60 a coil, 45 an injector, 36 a valve, lifter, rocker or camshaft, and 21 piston rings. 125 mention a recall, extension or settlement. 51 mention a flashing light. The NHTSA crash and fire flags are set on one report each, the towed flag on four; two injuries, no deaths.

Where it lands on the clock. Only 91 reports state a usable odometer: median 87,701 miles, quartiles 60,000 and 126,979.

50 of the 345, read in full

We drew 50 of the 345 at random (Python's random.sample, seed 20260925), read every narrative to its end, and coded what each report says the cause turned out to be.

First, the measurement problem. In 15 of the 50, P0304 appears only inside a code range or a list copied from a bulletin; the car itself stored P0301, P0302, P0303 or no code at all. A keyword count treats those as cylinder 4 failures. They are not.

In the 35 where the car really stored P0304:

The line that matters. 22 of 35 ended inside the engine; one ended at an ignition part. Eight of the 35 describe plugs, coils, an injector or a sensor fitted before the cause was found, and eight describe the engine stopping or losing power while moving. Ten are Fords, nine Hondas and six Kias.

The honest limits. One reader, one pass, no second coder; at 35 reports, a share near one half carries a margin of about 17 points either way. Complaints over-represent expensive outcomes — the owner whose $150 coil fixed a P0304 rarely writes to NHTSA — so this sample tells you what the bad cases look like, not how often a coil is the answer. The per-report coding is in the published file.

What owners say they paid, by lane

We read every dollar figure in the 345 complaints in context. Forty-two belong to this code's repairs.

These are owners' reports, not invoices. All 42, with ODI numbers.

Why P0304 comes back after new plugs and coils

A new coil and a new set of plugs is the most common first repair, and in the complaint file it is the most common one to fail. There are four usual reasons.

The misfire never followed the coil. Nobody did the swap test, or did it and bought the coil anyway.

Something is getting into cylinder 4. Coolant from a gasket or a crack, oil past rings, or water down the plug well. A new plug fouls again.

Cylinder 4 is not breathing. A collapsed lifter, a fallen rocker, a burnt valve or a lash adjuster that bleeds down cold. The spark is fine; the cylinder has no compression to fire.

The fuel is wrong on that cylinder. A leaking or clogged injector, or carbon on its intake valve.

Why does P0304 come back after replacing the spark plugs and coils? Usually because cylinder 4's spark was never the problem. In PulsCar's hand reading of a random 50 of the 345 federal complaints naming P0304, 35 described a car that really stored the code; one of those ended at an ignition part and 22 ended inside the engine — coolant or a head gasket, worn or stuck rings, lifters, valves or a failed engine. The manufacturers' documents say the same thing in their own terms: Ford's EcoBoost bulletins send dealers to a pressure test and a borescope, Honda's V6 extensions to the rings, GM's to the lifters on its deactivating cylinders, and Mazda's to a fallen rocker on cylinder 1 or 4. A coil swap test takes ten minutes; if the misfire stays on cylinder 4, the next money belongs to a compression or leak-down test, not another part.

Can you drive with P0304?

It depends on the light and on the coolant.

Steady light, smooth engine, coolant level holding. Drive gently and briefly, and do the swap test this week.

Flashing light, or shaking. Stop driving if you can. A flashing light means enough misfire to overheat the catalytic converter, and a converter is a much bigger bill.

P0304 with coolant loss, white smoke or a sweet smell. Park it. Coolant in a cylinder can hydrolock or scour the bore, turning a head gasket into an engine.

P0304 with a tick on a V8 or a Mazda with deactivation. Avoid highway runs until the valvetrain is checked; a failing lifter or rocker gets worse, not better.

Can you drive with a P0304 code, and how far? Only gently, and not at all if the light flashes or the coolant is dropping. A steady light with a smooth engine gives you days to test; a flashing light means the misfire rate can damage the catalytic converter within miles. In PulsCar's reading of a random 50 of the 345 federal complaints that name P0304, 11 described the engine stopping or losing power while the car was moving, and in the 35 that truly stored the code, 10 ended at coolant or a head gasket. Ford's EcoBoost bulletins and Honda's August 2026 head gasket extension both tie misfire codes to coolant loss and overheating. So check the coolant reservoir cold before you drive, avoid highways until a coil swap test and a pressure test have been done, and park the car if the light starts flashing.

Should you buy a used car with a P0304 history?

Only with the paperwork. A coil with a swap-test note is a closed case. A long block, short block or head gasket with a dealer invoice is a closed case, and possibly a better engine than the original. Three sets of plugs in a year is an open case. On a 2013-2017 Odyssey or 2013-2015 Pilot, ask whether the ring repair was done under Honda's extension. On a 2015-2019 EcoBoost, look at the coolant reservoir cold and ask for the service history. On a GM V8, listen at idle for a tick. And on any car, a cold start that stumbles for the first minute is worth a leak-down test before you pay.

Clearing P0304 before an inspection

Clearing the code resets the readiness monitors, and an inspection station will see an incomplete system or, if the fault is real, the code again. Misfire monitors return fast. And clearing can cost you coverage: a 2015 Odyssey owner was refused under Honda's extension "because the codes were not present that day." Keep the scan printout, the freeze frame and the repair order, and let the dealer see the code before anyone clears it.

What a shop actually does with a P0304, checkpoint by checkpoint

1. Reads every code, the freeze frame and the misfire counters, noting companion codes and cold versus hot.
2. Checks recalls, extensions and campaigns on the VIN.
3. Swaps the cylinder 4 coil with a neighbor and re-tests.
4. Pulls and reads the number 4 plug and looks down the well.
5. Checks the injector by swap, balance or flow where the engine allows.
6. Runs a compression or leak-down test, cold if the misfire is cold.
7. Pressure-tests the cooling system and borescopes cylinder 4 when coolant is dropping.
8. Inspects lifters, rockers and deactivation valves when there is a tick or a deactivation code.

Three P0304 traps that cost owners the most

Trap 1: the second set of coils. If the misfire did not follow the coil, a new coil cannot fix it. Lesson: ten minutes of swap test before any ignition part.

Trap 2: driving on falling coolant. On an EcoBoost or a Honda with a weakening head gasket, every mile with coolant in the cylinder adds damage. Lesson: P0304 plus coolant loss means park and pressure-test.

Trap 3: paying before checking the extension. Honda alone has at least four extensions that cover P0304 repairs on different models, and Ford has 21N12. Lesson: run the VIN, and ask for extensions by name, before the first invoice.

Three P0304 quotes, decoded

These are composite scenarios, not individual customers: each combines details that come up repeatedly in owner reports and shop quotes.

Scenario 1: 2017 Honda Odyssey, bought new in 2017, 131,000 miles, P0304 and P0302, quote $5,800 for "engine work." The owner asks the dealer to run the VIN: the van is inside 10 years from purchase and under 150,000 miles. The dealer finds oil-fouled plugs, cleans the pistons and replaces the rings under 26-001. Lesson: on a Honda V6, P0301-P0304 is a code range with a warranty attached.

Scenario 2: 2019 Ford Escape 1.5L, first sold in December 2019, 71,000 miles, P0304, coolant a little low, two coils already replaced. A five-hour pressure test and a borescope show coolant in cylinder 4. The car is inside 7 years and 84,000 miles of its warranty start, and 21N12 pays for the short block. Lesson: the coolant level is a diagnostic, not a top-up.

Scenario 3: 2019 Chevrolet Silverado 5.3L, 96,000 miles, P0304 with a light tick at idle, quote $400 for plugs and a coil. The shop runs the cylinder deactivation compression test: cylinder 4 is low. A lifter replacement on that bank costs about $4,000. Lesson: a tick plus P0304 on a GM V8 is a lifter question first.

The P0304 plan: 10 minutes, today, this week

In the next 10 minutes: read every code; check the coolant reservoir cold; if the light is flashing or coolant is low, stop driving; swap coil 4 with a neighbor if you can reach it.

Today: run the VIN on the manufacturer's owner site and ask a dealer for warranty extensions by name — Honda rings, injectors, carbon, head gasket; Ford 21N12; and look at the cylinder 4 plug.

This week: if the misfire stayed on 4, pay for a compression or leak-down test and, with coolant loss, a pressure test and borescope before any parts. Keep every printout; they are what an extension or a refund will ask for.

If the engine is ticking or knocking with the code and you are not sure where it comes from, PulsCar's free sound check screens a 30-second recording of your engine. It does not read codes; it is a second opinion on what the engine sounds like.

Sources and documents

Primary documents only. Where a claim in this guide is marked 🟢, it rests on one of the items below, and the reference is given so you can pull the document yourself. Claims marked ⚪ are shop-price estimates carried from PulsCar's repair guides — estimates, not measurements from a dataset we own.

1. Honda 25-062 and the July 2023 parts request (Civic Type R; NHTSA 11022041, 11024893, 10240328).
2. Honda 26-001, 26-002, 25-060, 25-061 and the 2025 owner letter (Odyssey and Pilot rings; 11028515, 11028516, 11020824, 11020825, 11021759); 13-078 (Soto settlement; 10099101); 13-055 (VCM; 10128532).
3. Honda warranty extensions: V6 software and injectors (10189943), 2015-2016 Fit injectors (10213565), 2018-2019 Fit carbon, 23-042 (10237804), 21-007 (10191759), head gasket, August 2026 (11036424).
4. Ford 19-2172 (2.0L, superseded by 19-2346 and 22-2229), 20-2100 (1.5L), Customer Satisfaction Program 21N12 (10162062, 10174400, 10213732).
5. Chrysler 09-002-14 Rev B, X56 (10139897).
6. GM PIP4568R, PIP5584, 23-NA-043, PIP5493 and PIP5493G (10094982, 10145466, 10235766, 10113881, 10144455); recall 25V-274.
7. Mazda SA-022/19 (supersedes SA-041/18) and the P3400 oil control valve alert (10160582, 10153295, 10201861).
8. Toyota T-SB-0010-24 Rev2 (11020670) and Lexus L-SB-0008-21 Rev1 (10198143).
9. PulsCar analysis of NHTSA public data, September 2026: 345 complaints naming P0304, 1,027 single-cylinder complaints by model, a random 50 read in full, 42 owner-reported amounts, 25 documents. Data and method.

For the neighboring codes: P0300 random misfire, P0301 cylinder 1, P0335 crankshaft sensor, P0420 catalyst, spark plug, fuel injector and head gasket costs, and the code index. For the cars: Escape, Edge, Fusion, Odyssey, Pilot, CR-V, Accord, Silverado and Wrangler.

Methodology. The complaint set is a case-insensitive whole-word search of NHTSA's complaint file for P0304 (and PO304); flags are computed by pattern and checked against the matched text. The by-model comparison uses only complaints naming exactly one cylinder code and not P0300. A random 50, drawn with a fixed seed, was read in full and its coding published; every complaint quoted, and every dollar figure, was read in full. Owner figures are not invoices. Nothing here is a failure rate.

Prices verified July to September 2026 in the repair guides they are carried from, and reviewed quarterly. Technical documents verified September 2026: the 23 documents marked as read in our published list — among them Honda 25-062, 26-001, 26-002, 13-055 and the August 2026 head gasket message, Ford 21N12, Chrysler 09-002-14 Rev B, GM PIP4568R, Mazda SA-022/19 and Toyota T-SB-0010-24 — were downloaded from static.nhtsa.gov and read in full on 25 September 2026; the recall record and the Honda Fit service article were read in NHTSA's published text the same day, when the complaint and bulletin files behind our analysis were also queried.

Had a P0304 fixed under an extension — or refused one? Email [email protected] with the vehicle, mileage and outcome.