P0421: Warm Up Catalyst Efficiency Below Threshold (Bank 1)
The warm-up catalytic converter on bank 1 does not clean the exhaust as well as required.
Normal risk
Usually safe to drive in the short term, but emissions are higher. Fix misfires first if present.
Short answer · Can I keep driving with P0421?
Usually yes. The car may fail an emissions test. Fix any misfire or rich-running code first, as they damage the converter.
What does P0421 mean?
Code P0421 is like P0420 but refers to the warm-up catalytic converter, a small converter close to the engine that lights off quickly after a cold start. The control unit compares the oxygen sensors before and after it.
What the car does when the code is set
The check engine light comes on, usually after the fault has been seen on two drive cycles in a row. The car drives normally, because the catalyst test does not change how the engine runs. Emissions are higher, and in many countries the car fails the emissions part of a roadworthiness test while the light is on.
How the system works
The catalytic converter sits in the exhaust and uses precious metals to turn carbon monoxide, unburnt fuel and nitrogen oxides into less harmful gases. To do that well, the engine keeps the mixture very close to the ideal air-fuel ratio, and the converter stores and releases oxygen as the mixture swings slightly rich and lean. An oxygen sensor before the converter controls the mixture, and a second one after it watches how much oxygen the converter is storing. The engine control unit runs this catalyst test once per drive cycle under steady conditions, usually after the engine has warmed up and the car has been driven at a constant speed for a while.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Worn catalytic converterAfter many years the coating inside the converter stores less oxygen and cleans the exhaust less well.
- Misfires or a rich mixtureUnburnt fuel overheats and damages the converter; this is often the real root cause.
- Faulty downstream oxygen sensorA slow or biased rear sensor can make a working converter look worn.
- Exhaust leakA leak before or near the oxygen sensors lets outside air in and distorts their readings.
- Engine burning oil or coolantOil and coolant residues poison the converter coating.
How it is diagnosed
- Read all stored codes and freeze-frame dataFix any misfire, fuel-trim or oxygen-sensor code first. A converter replaced without fixing them fails again.
- Check for exhaust leaksListen and look for leaks at the manifold, flexible joint and around the sensors with the engine cold.
- Compare both oxygen sensor signalsWith the engine warm, the front sensor switches quickly while the rear one should stay fairly steady. A rear signal that copies the front one points to a tired converter.
- Confirm before replacingA workshop may also check exhaust back-pressure and temperature across the converter. Use an approved converter that meets your car's emission standard.Exhaust parts get very hot. Let the car cool for at least an hour before touching them.
Questions about P0421
What is a warm-up catalyst?
A small catalytic converter mounted close to the exhaust manifold so it heats up quickly and cleans the exhaust soon after a cold start.
Will a fuel additive fix a catalyst code?
Additives cannot restore a worn converter coating. They may help only if deposits were the cause, and the code returns if the converter is worn.
How long can I drive with P0420 or P0430?
If the engine runs well, there is no immediate danger, but emissions are higher and an underlying fault may be damaging the converter further. Plan a diagnosis in the coming weeks.