P0421: Warm-up catalytic converter not cleaning well enough
Warm up catalyst efficiency below threshold (bank 1)
Severity: Monitor
P0421 is an emissions-monitor fault: the close-coupled converter is not cleaning the exhaust well during warm-up, and the car normally drives as usual.
It gets more serious if:
- Inspect soon: Have it inspected soon if misfire or rich-running symptoms are present, because Carista says a misfire or rich condition can damage the catalyst and should be fixed first.
P0421 in short
P0421 means the engine computer thinks the warm-up catalytic converter on bank 1 is not cleaning the exhaust well enough soon after a cold start. A worn converter is likely, but check for misfires, a rich mixture and the downstream oxygen sensor first. The car usually drives normally but will fail an emissions test.
Some engines have a small converter mounted close to the engine so it heats up quickly. The computer compares the oxygen sensor before and after it during warm-up.
If the downstream sensor starts to look like the upstream one, the converter is not storing and converting emissions, and P0421 sets.
Carista says the code often appears alongside P0420 as converters age, and that a misfire or rich fuel condition should be fixed before a converter is replaced.
Most likely causes
These are general causes for this code across vehicles. None is specific to one make or model.
Cause 1: Misfire or rich mixture that damaged or overloaded the converter
Common general cause- Why it ranks here
- Carista says to fix misfire or fuel-trim problems first; unburned fuel overheats and damages the catalyst.
- What would change this
- Rises if misfire codes or rich fuel trims are present. Falls if the engine runs cleanly with normal trims.
Cause 2: Faulty downstream oxygen sensor
Common general cause- Why it ranks here
- Carista and TaT both say the code depends on the downstream sensor reading, so a slow or lazy sensor can trigger it.
- What would change this
- Rises if the sensor waveform is slow or flat. Falls if the waveform switches normally.
Cause 3: Worn warm-up catalytic converter
Common general cause- Why it ranks here
- The converter ages and loses oxygen storage, which is the situation the monitor is designed to detect.
- What would change this
- Rises if misfires, trims and the oxygen sensor all check out. Falls if another cause is found.
Cause 4: Exhaust leak near the converter or oxygen sensors
Possible — needs testing- Why it ranks here
- TaT lists exhaust leaks as a common issue with this code.
- What would change this
- Rises if a leak is heard or seen near the manifold. Falls if the exhaust is sealed.
What to test first
Start at step 1. Each result tells you what it means and where to go next.
Step 1: Read all codes and fuel trims; note misfire, fuel-trim and oxygen sensor codes.
IfMisfire or fuel-trim codes are stored
It meansFix those first.
Points to: Misfire or rich mixture that damaged or overloaded the converter
IfOnly P0421 (maybe with P0420)
It meansCheck the downstream sensor.
Step 2: Compare upstream and downstream oxygen sensor waveforms during a cold start warm-up.
IfThe downstream sensor is slow or flat
It meansSensor fault.
IfSensors look normal and there is no leak
It meansThe converter is worn.
Symptoms
- Check engine light with no other change
- Failed emissions test
- Rotten-egg exhaust smell on some cars
How to fix
Step 1: Note whether the check engine light ever flashes and whether the engine misfires; if so, see a shop before driving far.
Parts and cost context
- Catalytic converter:Replace only after misfire, fuel trim and the oxygen sensor have been ruled out. Labour varies; get a quote.
Labor varies by vehicle and shop; get a quote before you approve a repair.
Compare used part prices on PartPriceGuideOn your vehicle
We don't have reviewed vehicle-specific evidence for P0421 yet. The causes and tests above apply across vehicles.
Related codes
P0421 questions
What is the difference between P0420 and P0421?
P0420 is the main catalyst efficiency code. P0421 is the warm-up catalyst version, checked during cold-start warm-up.
Sources and review
- Last reviewed
- Written by
- Saurabh K.
- Editorial review
- Pending
- Technician review
- Technician review pending
This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error