P0421 Warm-up catalyst efficiency below threshold (bank 1)
What P0421 means
The engine control unit compares the oxygen sensors before and after the warm-up catalytic converter on bank 1. The rear sensor is switching too much like the front one, which means the catalyst is storing too little oxygen and no longer cleans the exhaust properly.
P0421 is a generic SAE J2012 code: it means the same on every make. The exact cause still depends on the car, so read the freeze-frame data before replacing parts.
At a glance
- Severity
- Medium
- Typical repair cost
- 400–1800 €
- Workshop time
- 1–4 h
- Repair difficulty
- Hard
- DIY possible
- No
- Check engine light
- Yes
- Affects emissions test
- Yes
Parts and labour at an independent workshop in Europe. Main dealers charge more; the cause decides the bill, not the code.
Symptoms
- Check engine light on
- Usually no change in how the car drives
- Possible sulphur (rotten egg) smell
- Failed emissions test at technical inspection
Common causes
- Worn or contaminated catalytic converter
- Faulty downstream oxygen sensor
- Exhaust leak near the oxygen sensors
- Misfires or rich running that damaged the catalyst
- Oil or coolant burning that has coated the catalyst
Can I keep driving with P0421?
You can keep driving, but have it checked within one to two weeks. The car may fail its technical inspection, and any misfire must be fixed first to protect a new catalyst.
What to check first
- Check for misfire, fuel trim or oxygen sensor codes and fix those first
- Look for exhaust leaks ahead of and around the downstream sensor
- Compare front and rear oxygen sensor signals in live data at steady speed
- Replace the downstream sensor if it is lazy, otherwise replace the catalytic converter
Related codes
- P0420 Catalyst system efficiency below threshold (bank 1)
- P0430 Catalyst system efficiency below threshold (bank 2)
- P0411 Secondary air injection system incorrect flow detected
- P0431 Warm-up catalyst efficiency below threshold (bank 2)
- P0410 Secondary air injection system A
- P0406 EGR sensor A circuit high
- P0405 EGR sensor A circuit low
- P043E EVAP leak detection reference orifice low flow
Data: TORKO fault-code catalogue, built on OBDex (CC0-1.0) and reviewed against workshop practice. Informational only — it does not replace a diagnosis by a mechanic.
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