Catalytic converter
P0420
Catalyst System Efficiency Below Threshold (Bank 1)
What does P0420 mean?
P0420 means your engine computer has decided the catalytic converter on bank 1 is no longer cleaning exhaust gases as efficiently as it expects. It compares two oxygen sensors, one before the converter and one after. When those two readings start to look alike, the converter is no longer doing its job.
Can I keep driving with P0420?
Moderate urgency
Usually safe to keep driving in the short term, but it will fail an emissions test and an untreated root cause can destroy a converter that was merely being reported on.
Symptoms of P0420
Check engine light on, usually with no change in how the car drives
Failed emissions or smog test
Slightly worse fuel economy in some cases
Sulphur or rotten-egg smell from the exhaust
Common causes of P0420
A genuinely worn-out catalytic converter, most often on higher-mileage vehicles
A failing downstream oxygen sensor reporting badly
An exhaust leak upstream of the rear oxygen sensor
Unburned fuel reaching the converter because of a misfire or a rich mixture
These are ordered roughly by how often they turn out to be the cause — but the code names a system, not a part. Confirming which one is yours takes data from the running engine.
What does it cost to fix P0420?
$200 – $2,500
Typical US independent-shop range, parts and labour included. It is wide on purpose: the cheap end is usually a sensor, a hose or a cap, and the expensive end is the major component the code was reporting on. Which one you need is a diagnosis, not a guess — and dealer rates run higher than these figures.
What does P0420 mean for your car?
This page explains what P0420 means in general. It cannot tell you which of those causes is yours, because that answer is in your car's own data. Kaiju Mechanic reads the codes your car is actually storing, plus live sensor data through any standard ELM327 adapter, and interprets it against your exact year, make, model and mileage. You can also just ask it about P0420 in chat — no adapter needed for that.
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