Warm-up catalyst efficiency below threshold (Bank 1)
What P0421 means
Your car's computer set P0421: Warm-up catalyst efficiency below threshold (Bank 1). It belongs to the emission controls group of codes. It is a generic code, so it means the same thing on every make. The check engine light usually comes on once the fault has been seen on two drives in a row. Until then it shows up as a pending code.
Can I keep driving? Usually safe to drive. The car may run normally, but it will not pass an emissions test until this is fixed.
Common causes
- Worn-out catalytic converter
- Failing downstream (after-cat) oxygen sensor
- Exhaust leak near the oxygen sensors
- An old misfire or oil burning that damaged the converter
Fix the cheapest, most likely cause first. Clearing the code without a repair only turns the light off until the fault comes back.
Check it yourself
Fix any misfire codes first. On the Graph tab, the downstream O2 sensor should stay fairly steady. If it switches as fast as the upstream one, the converter is not storing oxygen.
Torque Deck reads P0421 with a cheap Bluetooth OBD2 adapter. It also shows the freeze frame (engine conditions at the moment the code set) and the live data you need to narrow it down.
Parts
Parts most often replaced for P0421: catalytic converter. Confirm the cause first, and match the part to your exact engine.
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Related codes
This page explains what the code usually means. It is not a diagnosis of your car. When in doubt, ask a mechanic.