P0420
P0420 on a Honda
Catalyst system efficiency below threshold (Bank 1)1
What does P0420 mean?
The engine computer has judged the catalytic converter on Bank 1 as working below its efficiency threshold. It checks this by comparing the oxygen sensor before the converter with the one after it.4
Can I keep driving?
With a steady check engine light, Honda's manual says to avoid high speeds and get the vehicle inspected immediately. If the light flashes, stop safely.2 Flashing light: what to do →
What should I check first?
Every other stored code. A misfire or fuel-mixture code alongside P0420 changes the diagnosis — fix those first.
Diagnosis
5 steps, in order
- Read every stored code and the freeze-frame data. Write them all down before you clear anything.
- Fix misfire and fuel-mixture problems first — Honda's manual warns that a misfire can damage the catalytic converter.2 Repair those, clear the codes, drive normally and check again.
- Check for exhaust leaks ahead of the rear oxygen sensor.
- Compare front and rear oxygen sensor live data.
- Only then consider the catalytic converter.
P0420 or P0430? Same fault, different side of the engine: P0420 is Bank 1 (the side with cylinder 1), P0430 is Bank 2 — V6 engines only, because a 4-cylinder has one bank.1
Tests
Live sensor data, exhaust leaks
Compare the oxygen sensors. The engine computer judges the converter by comparing the sensor before it with the sensor after it.4 Watching both side by side in live data shows whether the rear sensor is simply copying the front one.
Look for exhaust leaks ahead of the rear oxygen sensor: a leak lets outside air reach the sensor and can skew the reading the converter is judged on.
Possible causes
Why P0420 is set
Tools
What helps each test
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Possible repair
Only after the tests
Don't replace the converter based on the code alone. It is an expensive part, and P0420 can be set by something else — a misfire, a fuel-mixture problem, an exhaust leak or a sensor.
Parts are shown only with a confirmed fit: your car's details from Honda's owner's manual or service information, and the part's fit from the part maker's own application catalog. No converter is listed yet — none has a confirmed fit for a specific Honda.
Sources
Honda manual, SAE J2012, Innova
- 1 SAE J2012 — Diagnostic Trouble Code Definitions (P0420, P0430).
- 2 2014 Honda Accord Sedan Owner's Manual — pp. 67 (misfire makes the lamp blink), 79 (a misfire can damage the catalytic converter), 519 (what to do when the lamp comes on or blinks).
- 3 2014 Honda Accord Sedan Quick Start Guide — p. 4 (malfunction indicator lamp shown amber).
- 4 Innova CarScan Inspector 5310 Owner's Manual — pp. 16–17 (how the catalyst monitor uses the sensors before and after the converter), p. 37 (Live Data).
Owner's manual references are to the model and year named above. Wording and procedures can differ on your Honda, so check your own owner's manual.