P0421 — Warm Up Catalyst Efficiency Below Threshold (Bank 1)
Moderate severityWhat it means
P0421 is stored when the engine control unit sees a fault involving the bank 1 warm-up catalytic converter. The warm-up catalyst sits close to the engine and is monitored by the oxygen sensors; this code means its measured efficiency on bank 1 has dropped below the threshold.
Common symptoms
- Usually only a check engine light
- A slight loss of power or economy
- Failed emissions inspection
Likely causes
- An aged or failing warm-up catalytic converter
- A faulty oxygen sensor or an exhaust leak skewing the reading
What to check first
- Resolve any misfire or fuel-trim codes first
- Compare upstream and downstream oxygen-sensor signals in live data
Can you keep driving?
Generally drivable, but address it soon: the cause can worsen and the car is likely to fail an emissions test.
Frequently asked questions
What is the most common cause of P0421?
The most likely cause is: An aged or failing warm-up catalytic converter. Several other faults can set P0421 as well, so confirm the real cause with live data and the checks above before replacing any parts.
Can I just clear P0421 and keep driving?
Clearing P0421 with an OBD2 scanner turns the warning light off, but if the underlying fault has not been repaired the code will usually come back within a few drive cycles. Fix the cause first, then clear the code to confirm the repair worked.
How serious is P0421?
P0421 is a moderate-severity code. The car is often still drivable, but have it diagnosed soon — the underlying problem can get worse and it may fail an emissions test.
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