Which Check Engine Codes Get a Software Fix? 1,412 Manufacturer Bulletins Analyzed

When a check engine light comes on, the conversation at the counter is almost always about parts: a converter, a sensor, a thermostat. The manufacturers' own bulletins tell a different story for some codes. We read every one of them.

On 17 September 2026 we took NHTSA's public file of manufacturer communications received from 2015 to August 2026 and pulled every document whose summary names one of 17 common check engine codes. We merged revisions and brand duplicates and removed helpline lists, dealer messages and owner letters that repeat or precede a numbered bulletin. That leaves 1,412 distinct bulletins from 15 manufacturer groups. We then asked one narrow question the file can answer reliably: how many of them are software updates?

The short version

What did the Pulscar Check Engine Code Index find? That software is a far bigger part of how manufacturers fix check engine codes than a parts counter suggests, but not evenly. Of 1,412 distinct bulletins received by NHTSA from 2015 to August 2026 that name one of 17 common codes, at least 639, or 45 percent, describe a reprogram, reflash, calibration or logic update. Stellantis publishes many separate flash bulletins, so we report every share without it too: the floor is then 213 of 911, or 23 percent. The code that stands out either way is P0128, coolant temperature below thermostat regulating temperature: at least 74 percent of its 125 bulletins are software, and 52 percent without Stellantis. At the other end are evaporative leaks. Outside Stellantis, P0442 and P0455 together have two software bulletins in 76. Underboost, camshaft timing and low fuel pressure codes sit between 10 and 17 percent.

How did Pulscar count software updates in manufacturer bulletins? With a published rule applied to the summary NHTSA stores for each bulletin, then checked by hand in both directions. A bulletin counts when its summary says it reprograms, reflashes or updates the engine, transmission or ignition control module, changes a calibration, revises control logic, or starts with the word Flash, as Stellantis flash bulletins do. We read 30 randomly chosen flagged bulletins: 28 were plainly software updates and the other two made reprogramming conditional on a part number or calibration table check. We also read 40 randomly chosen bulletins the rule did not flag. Two were clearly software, Kia campaign SC114 and GM PIP5905, and 14 did not say what the repair was. That is why every share on this page is a floor, not an estimate.

Does a Pulscar index software share mean a code is usually just a software problem? No, and the difference matters when you are standing at a service desk. A bulletin applies to particular models, engines, build dates and symptoms, and many software bulletins also require an inspection or a part. The index measures what manufacturers wrote to dealers, not how often a repair succeeds and not how often a part fails. What it does support is a practical rule. For codes with a high software share, especially P0128 and to a lesser degree P0420 and P0171, ask the dealer to run your VIN for open bulletins and software updates before approving a thermostat, converter or sensor. For codes with a low share, such as P0442, P0455 or camshaft timing codes, the fix is far more likely to be physical, and a proper test matters more than an update.

The index

Seventeen codes, chosen because they are among the most searched OBD-II codes or appear most often in our guides. Software, at least is the share of distinct bulletins our rule flags. Without Stellantis removes Chrysler, Dodge, Jeep, Ram, Fiat and Alfa Romeo bulletins. Coverage counts bulletins whose summary mentions special coverage, a warranty extension or an emissions campaign. The last three columns come from NHTSA's complaint file: complaints naming the code, median reported mileage and the code most often named alongside it.

A bulletin that names several codes appears in each code's row. Without Stellantis, P0441 has 17 bulletins and P0401 has 31, so treat those shares as rough. Median mileage is the owner-reported odometer in complaints that give one; for P0174, P0401 and P0442 fewer than 15 complaints report mileage.

Finding 1: P0128 is the code manufacturers fix with software most often

P0128 means the engine computer expected the coolant to reach a warmed-up temperature and it did not. The instinctive repair is a thermostat, and sometimes that is right. But in the bulletin file, at least 92 of 125 P0128 bulletins are software updates, and the pattern is not a Stellantis artefact: without Stellantis, 32 of 62 still are, led by Hyundai with 10, Kia with 9, Ford with 5 and Mitsubishi with 4.

The reason is built into the code. P0128 is a judgement the computer makes from a model of how fast an engine should warm up, and manufacturers revise that model. Ford TSB 25-2486 says a P0128 lamp "may be due to the software in the PCM." Mitsubishi campaign SC-26-003 reflashes the engine computer for P0128. GM's 22-NA-054 corrects a calibration timing error on 2021 crossovers; our rule does not flag it, which is one more reason the share is a floor.

Software is not the whole story. GM's special coverage N252508350 replaces the thermostat on certain 2021 XT5, XT6, Blazer and Acadia models for 15 years or 150,000 miles; its NHTSA summary does not name the code, so it is not in the count. The index says to ask about an update first, not that a thermostat never fails. Our P0128 guide walks through both.

Finding 2: emissions-monitor codes lean software, but check who is publishing

P0420, P0430, P0171, P0101, P0401, P0441 and P0456 all come from monitors that compare sensor readings with expected values, and all show software shares of 46 to 74 percent overall. Much of that falls away without Stellantis, which publishes many separate flash bulletins that each list dozens of codes. P0456 drops from 72 to 21 percent.

What survives is still useful. Without Stellantis, about one P0420 bulletin in three and one P0171 bulletin in three are software updates, and P0101 stays at 38 percent. Examples we have opened and read:

• GM 18-NA-331: reprograms the ECM on 2013-2015 Trax and Encore 1.4L models for P0420.
• Ford TSB 25-2188: a PCM software update for P0420 on 2022-2024 Explorer 2.3L models.
• Ford TSB 23-2195, superseded by 25-2342: PCM software for P0171 on 3.3L F-150s.
• Honda 20-100: for P0420 or P0430 alone on the 2016 Pilot, update the software and test the injectors, with the warranty on the repair extended to 10 years or 150,000 miles.

The P0420 and P0171 guides explain the non-software causes, which are still the majority.

Finding 3: leaks and mechanical faults are fixed with parts

Where a code describes a physical condition the computer measures directly, software is rare.

Evaporative leak codes are the clearest case. Outside Stellantis, P0442 has one software bulletin in 36 and P0455 one in 40. The bulletins are about caps, filler necks, canisters, valves and test procedures: Kia TSB FUE040 replaces a canister close valve that sticks open on the Rio, and GM 23-NA-073 addresses dust getting into the evaporative canister. Our P0442 and P0455 guides cover how technicians find the leak.

Boost, camshaft timing and fuel pressure codes sit between 10 and 20 percent without Stellantis: P0299 at 10, P0011 at 12, P0087 at 17, P0016 at 20. Ford TSB 25-2200 traces misfire codes with a ticking noise to worn camshafts and tappets, and GM's P0016 information tells technicians to check whether the crankshaft reluctor has moved. The misfire codes, P0300 and P0301, fall to 20 and 17 percent.

Finding 4: where the warranty extensions are

27 of the 1,412 bulletins mention special coverage, a warranty extension or an emissions campaign in their summary. That is a floor too: GM's thermostat coverage N252508350, for one, does not name a code in its summary. By code, P0301 has 7, P0420 has 6, and P0171, P0101 and P0300 have 5 each. Three patterns stand out:

• Misfires from piston rings. Honda bulletins 13-078 to 13-082 cover rings that rotate and foul spark plugs on several 2008-2013 models; all five name P0301.
• Catalyst codes the computer gets wrong. Honda 18-022 and 18-023 extend the warranty on 2006-2007 Accord Hybrids whose ECM misreads sensor inputs as converter deterioration and sets P0420 or P0430. GM special coverage 15821 covers an exhaust clamp leak that sets P0420 on 2013 models for 8 years or 80,000 miles.
• Lean codes from the PCV valve. GM special coverages N202299080, N192210230 and N192291620 replace camshaft covers with cracked PCV diaphragms on 1.4L and 1.6L engines; N202299080 alone lists about 810,404 vehicles, covered for 10 years or 120,000 miles.

For a driver, that is the second question to ask after software: is there coverage for this code on my VIN?

Finding 5: what the complaint file adds

NHTSA's complaint file is a different kind of evidence, owner reports rather than dealer instructions, and it adds two things the bulletins cannot.

When the codes show up. Among complaints that report mileage, P0420 has the highest median, 109,057 miles across 98 complaints, consistent with a converter-efficiency monitor that trips late in a car's life. P0299 underboost, 65,000, and P0128, 67,650, show up earlier.

Which codes travel together. P0420 and P0430 are named together in 24 complaints, P0171 and P0174 in 15, P0300 and P0301 in 94, P0016 and P0017 in 42. A pair like that usually points to one shared cause rather than two separate failures, which is the logic our guides use.

How to use this if your check engine light is on

Get the exact code, not the category. A free scan gives you the code; this index is keyed to it.

For P0128 and the catalyst and lean codes, ask about updates before parts. Ask the dealer to check your VIN for technical service bulletins and pending software updates. Updates on emissions-related systems are often done at no cost when a bulletin or campaign applies, but that depends on the bulletin and your warranty.

For leak, timing, boost, fuel-pressure and misfire codes, insist on a test. A smoke test, a fuel pressure reading, a compression or leak-down test. The bulletins for these codes are mostly about parts and procedures.

Listen as well as read. Misfires, timing chain rattle and failing fuel pumps often make themselves heard before or alongside a code; our study of car noises that come with no warning light found that most owners who describe a noise report no warning at all.

Free, no card, no subscription. The report comes by email.

What this index does not show

It is not a failure rate or a reliability ranking. A bulletin is a manufacturer communication. Makers publish at different rates and in different formats; Stellantis alone accounts for 501 of the 1,412 bulletins.

It does not say a software update will fix your car. Bulletins apply to specific vehicles and conditions, and many combine software with inspection or parts.

The software share is a floor. It is read from NHTSA's summary of each bulletin, not the full document. In our check of unflagged bulletins, about one in three summaries did not say what the repair was.

Other remedy types are not reported. We tried to classify replacements and inspections from the summaries as well, and a hand check put that classification at roughly three correct in four, too low to publish.

Complaint numbers are small for some codes and are unverified owner reports.

Methodology

Source. NHTSA's manufacturer communication flat files TSBS_RECEIVED_2015-2019, TSBS_RECEIVED_2020-2024 and TSBS_RECEIVED_2025-2026, downloaded in September 2026, using the document ID, communication date, make, model, model year and summary fields. Latest communication date in the index: 31 August 2026.

Inclusion. The summary names the code as a whole word. Removed before counting: heavy-truck, bus, motorcycle and RV makers; 205 helpline analysis entries and documents marked not applicable to US-market vehicles; 138 Honda and Acura dealer messages, parts-collection requests and owner notification letters, which announce, investigate or repeat a numbered bulletin rather than describe a repair; and 11 entries with no document number.

One bulletin, counted once. A bulletin is one manufacturer group plus one normalized document ID. Brands of the same group share IDs (Buick, Cadillac, Chevrolet and GMC; Chrysler, Dodge, Jeep and Ram), and revision and version marks such as REV A, a trailing letter or a Volkswagen _2 suffix are removed, so PIP5110, PIP5110A, PIP5110B and PIP5110C count once. That leaves 2,151 code-bulletin pairs and 1,412 distinct bulletins.

Software rule and audits. The exact pattern, the excluded makers and the grouping rules are in the published method file. Of 30 randomly selected flagged bulletins, 28 were plainly software updates and 2 made reprogramming conditional. Of 40 randomly selected unflagged bulletins, 2 were clearly software updates and 14 did not state a remedy.

Complaints. 777,040 distinct vehicle complaints received by NHTSA from 1 January 2015 to 8 September 2026, matched on the exact code in the complaint text.

Download the data

• Index summary, one row per code (CSV)
• All 1,412 bulletins with document IDs, dates, model years and flags (CSV)
• Exact rules, exclusions and audit notes (TXT)

Licence. CC BY 4.0. Please credit Pulscar Check Engine Code Index and link to this page.

For reporters. If you need a cut we did not publish, one manufacturer, one model year band, or a code not in the 17, email [email protected] and we will run it with the same rules.

Code guides

Each of these uses the index alongside manufacturer documents: P0128, P0420, P0171, P0300, P0455 and P0442. For the light itself, start with check engine light on.

Sources and documents

Primary sources only.

1. NHTSA manufacturer communication flat files — received 2015 to 2026, with field dictionary. Field dictionary
2. NHTSA ODI complaint flat files — 777,040 distinct vehicle complaints, 1 January 2015 to 8 September 2026. Field dictionary
3. Ford TSB 25-2486 — PCM software, P0128. NHTSA summary in the bulletin file.
4. Mitsubishi SC-26-003 — ECU reflash for P0128, P2096 and P050A. NHTSA summary in the bulletin file.
5. GM 22-NA-054, March 2022 — 2021 calibration, P0128. PDF
6. GM special coverage N252508350, 21 October 2025 — thermostat, 15 years or 150,000 miles. PDF
7. GM 18-NA-331, November 2018 — ECM reprogram, P0420. PDF
8. Ford TSB 25-2188, May 2025 — Explorer 2.3L PCM software, P0420. PDF
9. Ford TSB 23-2195 and 25-2342 — F-150 3.3L PCM software, P0171. 23-2195 · 25-2342
10. Honda SB 20-100, November 2020 — 2016 Pilot, software and injectors, 10 years or 150,000 miles. PDF
11. Honda SB 18-022 and 18-023, June 2018 — Accord Hybrid catalyst warranty extension, P0420 and P0430. NHTSA summary in the bulletin file.
12. GM special coverage 15821, 2016 — exhaust clamp, P0420, 8 years or 80,000 miles. NHTSA summary in the bulletin file.
13. GM special coverage N202299080, July 2020 — camshaft cover and PCV, about 810,404 vehicles, 10 years or 120,000 miles. PDF
14. Ford TSB 25-2133 and Volkswagen 01-21-02 — software bulletins the rule does not flag. 25-2133 · 01-21-02
15. Pulscar index files — summary, bulletin list, method and audits. Method

How these numbers were built: one script over NHTSA's public bulletin and complaint files, run on 17 September 2026, with the inclusion rules, grouping, software pattern, audits and limits described above and in the published method file.

Prices verified: not applicable, this index quotes no repair prices · Technical documents verified: 17 September 2026 · Data refreshed: 17 September 2026

Re-ran the rules and got a different number? Email [email protected] with your counts and file dates.