P0420 on Honda: Bulletins, Warranty Extension and Fixes

What makes P0420 different on Honda?

The code means the same thing on a Honda as on any other car. The powertrain control module (PCM) compares the oxygen sensors before and after the converter, and sets P0420 when the converter looks too weak. Our main P0420 guide explains that test in detail.

What is different is how often Honda has documented a cause other than a worn converter. In a March 2021 owner notice filed with NHTSA, American Honda said some 2016 to 2017 Pilot, 2017 Ridgeline and 2018 Odyssey models may turn on the check engine light with P0420, P0430, misfire codes P0300 to P0306, or air-fuel variation codes P219A and P219B. Depending on the codes, the PCM and transmission software may need an update and the fuel injectors may need replacing. Honda extended the warranty on these repairs to 10 years from the original purchase date or 150,000 miles, whichever comes first, and the coverage transfers to later owners ^[1].

You can read the notice yourself on NHTSA's document server, static.nhtsa.gov. The technician bulletin behind that notice, Service Bulletin 20-100, gives two reasons. The PCM software may misinterpret sensor inputs as a deteriorated catalytic converter, and debris from the high pressure pump or the injector machining process can cause wear or clogging inside one or more injectors ^[2].

Which Honda models have a P0420 bulletin?

These NHTSA-hosted Honda and Acura documents mention P0420. Your VIN decides whether one applies, so treat this as a starting list, not a diagnosis.

Bulletin	Vehicles	What Honda says	Corrective action
SB 20-100, warranty extension	Some 2016 to 2017 Pilot, 2017 Ridgeline, 2018 Odyssey	PCM software may misread sensors; injector wear or clogging	Software update, cylinder air-fuel test, injectors if needed
SB 22-045	2014 to 2015 Civic Hybrid	Sensors may misread the catalyst as deteriorated	PGM-FI software update
SB 22-040	2014 to 2016 CR-Z	Catalyst may be misread, or may really be worn	Software update, test drive, check the warm-up converter
Acura SB 19-049	2015 to 2017 TLX with V6	Software may misread a deteriorated converter	PGM-FI software update

Sources for the table: SB 22-045 ^[3], SB 22-040 ^[4] and Acura SB 19-049 ^[5]. The CR-Z bulletin adds that a need for parts will be unlikely after the software update and test drive, which is a good reminder to try the cheap fix first.

Not sure which engine or trim you have? Our VIN decoder shows the basic build details.

Is it safe to drive a Honda with P0420?

Usually yes, in the short term. P0420 rarely changes how the car drives, so you can normally get to a dealer or shop. Honda's notice warns that a vehicle with the light on may fail a state or local emissions inspection ^[1], and California's Smog Check will not pass a car with the check engine light on ^[7].

There is one Honda-specific reason to pay closer attention. On the models in the warranty extension, P0420 can travel with misfire codes. If the check engine light flashes or the engine shakes, pull over and stop driving, because a misfire sends unburned fuel into the converter and can overheat it. See P0300 and our guide to a flashing check engine light.

How do you check P0420 on a Honda?

Start cheap, and let Honda's own records do some of the work.

1. Read every code and save the freeze frame. A scanner that shows live data and freeze frame is enough; see how to use an OBD2 scanner. Misfire or P219A/P219B codes alongside P0420 match the pattern in SB 20-100.
2. Check whether your model appears in a bulletin. Use the table above, then confirm with a dealer.
3. Ask a Honda dealer to run the VIN. Honda's bulletins say not to assume a bulletin applies and to contact a dealer to confirm. SB 20-100 tells dealers to check eligibility by VIN ^[2].
4. If a bulletin applies, the software update and the listed tests come first. Under SB 20-100, P0420 or P0430 alone gets a software update and a cylinder air-fuel test, and the injectors are replaced only if the test fails.
5. If no bulletin applies, follow the normal path: fuel trims and oxygen sensor data, exhaust leaks, the rear oxygen sensor, then the converter. Identifix also recommends checking for PCM updates and bulletins by VIN before replacing a converter ^[6].

Software updates, injector replacement and converter work are dealer or qualified technician jobs. Your part is collecting the codes and asking the right questions.

How are Honda P0420 causes different from the usual ones?

The usual causes still apply to Hondas: a worn converter, a faulty rear oxygen sensor, exhaust leaks, misfires and an engine burning oil or coolant. Honda's documents add two items that matter more on the listed models.

The injector link explains the odd mix of codes in SB 20-100. P219A and P219B mean the air-fuel ratio varies between cylinders on the rear bank or the front bank. A worn or partly clogged injector feeds one cylinder a different mixture from its neighbors, which can show up as an imbalance code, a misfire, or a converter that looks weak to the PCM ^[2]. That is why the bulletin calls for a cylinder air-fuel test before injectors are replaced, and why a new converter alone would not cure the problem.

	Generic P0420	Honda bulletin cases
Most likely cause	Worn converter, sensor or leak	PCM software, injectors on listed V6 models
Other codes often present	Misfire or fuel trim codes	P0430, P0300 to P0306, P219A, P219B
First fix	Diagnose, then repair the cause	Dealer software update and Honda's tests
Who pays	Owner, unless in emissions warranty	Honda, if the VIN is covered and the cause matches

What does a Honda P0420 repair cost?

It depends mostly on coverage. If your VIN is in the warranty extension and the dealer finds the codes are related to the PCM software or the injectors, Honda says the repairs are free within 10 years or 150,000 miles, and owners who already paid for those repairs may be eligible for reimbursement ^[1].

If you think you qualify for reimbursement, keep the repair order, the invoice and a printout of the codes. Honda's notice points owners to its reimbursement instructions and to American Honda's customer support line, and a dealer can confirm whether your VIN was part of the extension.

Outside that program, federal rules give catalytic converters an 8-year or 80,000-mile warranty period on light-duty vehicles ^[8]. Past both, the cost depends on what failed: a software update is a short labor job, while injectors or a converter cost far more. We have not found a Honda-specific price survey we trust, so ask for a written estimate that names the failed part and the test that found it.

After any repair, clearing the code does not fix anything, and inspection stations check readiness monitors. Never consider removing or disabling emissions equipment; tampering is illegal under the Clean Air Act ^[9]. Other Honda codes? Try our code lookup.