OBD2 codes.
A code identifies a signal or system that needs to be checked, not a part you should replace.
Got an OBD2 Bluetooth adapter? Read the codes straight from your car.1 code entry · 0 related faults
P0421The warm-up catalyst is not cleaning enough (bank 1)Urgency low
Warm Up Catalyst Efficiency Below Threshold (Bank 1)
Some engines have a small catalyst near the manifold that starts working soon after starting. Two lambda sensors, before and after it, check that it works.
What the car has detected
The sensor after the catalyst changes almost like the one before it: the catalyst no longer stores oxygen as it should.
It is not dangerous, but emissions rise and it may fail the inspection. If the engine misfires, fix that first: misfires can damage the catalyst.
Symptoms
- Engine warning light on
- Rotten-egg smell in some cases
- May fail the emissions test
Possible causes
- Worn or damaged catalytic converter
- Faulty lambda sensor after the catalyst
- Exhaust leak ahead of the sensors
- Misfires or oil burning that have damaged it
Checks for the garage
- Compare the two lambda sensors in live data
- Look for exhaust leaks
- Check there are no other misfire codes
Next step
Ask for the lambda sensors and any exhaust leaks to be checked before the catalyst is replaced.
You can keep using the car and book a visit without hurry, unless the light flashes: then go soon.