P0421: the warm-up catalyst isn't cleaning up well enough
Some exhaust systems have a warm-up catalyst: a smaller catalytic converter fitted close to the engine so it heats up and starts cleaning the exhaust sooner than the main converter further back. This code means the engine control unit's check of bank 1's warm-up catalyst shows it isn't cleaning the exhaust as effectively as the computer expects. It's a statement about the exhaust's performance, not automatically a statement about which part has failed.
You might notice
- No symptom you'd feel while driving, beyond the engine warning light coming on
- warm-up catalyst
- — a smaller catalytic converter fitted close to the engine so it heats up and starts cleaning the exhaust sooner than the main converter further back
- catalytic converter
- — the part of the exhaust that cleans up the gases before they leave the car
- bank 1
- — the side of the engine that has cylinder number 1
What the code itself says
P0421 is a powertrain (engine, gearbox and drive system) code, and the 0 after the letter means it is a generic code with a standard meaning.
Grouped here as: Engine running, fuelling and misfires — Air metering, fuelling, ignition, misfire, idle, boost, knock, oxygen sensors and the catalyst, all working together in the running engine. That grouping is ours, to help explain the code. The code's official family is the letter at the start.
This page is generic. Your car isn't.
A code on its own only tells you what the sensor saw, not what's actually wrong — and that changes next to your other codes. Read this one together with the rest of your scan and we'll work out what's really going on.
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