P0421
Warm Up Catalyst Efficiency Below Threshold (Bank 1)
The pre-cat (warm-up) catalyst on bank 1 has lost too much oxygen storage capacity. Its primary purpose — quickly reaching light-off after a cold start — is no longer met within the OBD limits.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: hard
Symptoms
- Check engine light on
- Failed emissions inspection
- Possibly slightly increased fuel consumption
- Sulfur (rotten egg) smell from exhaust under load
Common causes
Ordered by how often each turns out to be the cause.
- Pre-catalyst aged or damagedCommon
- Oxygen sensor driftPossible
Parts involved
- Warm Up Catalyst
- Oxygen Sensor Upstream
- Oxygen Sensor Downstream
P0421 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.