Trouble codes › P0421
P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1)
Emissions
P0421 means the small warm-up catalytic converter on bank 1 isn't cleaning the exhaust well enough.
Like P0420, but for the small warm-up catalytic converter close to the engine on bank 1: the engine computer decided it isn't cleaning the exhaust well enough.
Usually safe to drive for now, but it will fail an emissions test. Fix any misfire first to avoid ruining the converter.
Common causes
- A worn-out warm-up catalytic converter
- An exhaust leak near the sensors
- A failing oxygen sensor
- Misfires, oil or coolant burning damaging the converter
Symptoms
- Check-engine light
- Failed emissions test
What to check first
- Fix misfire and fuel trim codes first.
- Check for exhaust leaks.
- Compare front and rear oxygen sensor readings.
- Check the emissions warranty on the converter.
What does P0421 mean on your vehicle?
The likely cause depends on your engine. iRepair researches your exact year, make, model and engine (service bulletins, recalls and owner reports) and tells you what to check first. Your first 3 questions are free.
Ask iRepair about P0421No scanner? Most auto parts stores read check-engine codes for free.
Related codes
General guidance for P0421 on most vehicles. Some makers use the same code a little differently; confirm in the service information for your vehicle, put safety first, and see a professional when in doubt.