P0421: The warm-up catalytic converter is not cleaning exhaust well
Warm Up Catalyst Efficiency Below Threshold (Bank 1)
Yes, you can drive for now. The car will run normally. Avoid driving if the light is flashing, which indicates a misfire that will accelerate converter damage.
What it means
Some vehicles have a smaller warm-up catalyst, also called a pre-cat, mounted close to the engine to heat up and clean exhaust quickly after a cold start. P0421 means this converter is no longer cleaning exhaust efficiently on Bank 1. Like P0420, this code is commonly set because of a failing oxygen sensor upstream or downstream of the pre-cat, not necessarily a dead converter. The pre-cat also runs hotter than the main converter, so it tends to fail at lower mileage on vehicles that see lots of short trips.
Common causes
- Worn or failing warm-up catalytic converterPre-cats run very hot and fail sooner than main converters, especially on vehicles driven mostly short distances.
- Failing oxygen sensor near the warm-up catA slow or biased sensor can make a healthy pre-cat appear inefficient. Rule this out first.
- Exhaust leak between the sensor and the converterFresh air entering through a leak skews the oxygen readings.
- Coolant or oil contamination of the converterA head gasket leak or worn rings can poison the pre-cat rapidly.
What it costs
- Oxygen sensor replacement$150 to $350
- Exhaust leak repair$100 to $300
- Warm-up catalytic converter replacement$500 to $1,800 installed
Symptoms
- Check engine light on, steady
- Often no noticeable change in driving
- Possible rotten-egg smell
- Failed emissions test
- Slightly reduced fuel economy
What to do next
Check for misfire codes first (P0300 series). If any are present, fix them before touching the converter or the code will return. Inspect the oxygen sensors around the warm-up cat and test their response. If the sensors are healthy and there are no other codes, the pre-cat is likely worn out.
Lewis shows the oxygen sensor readings around the warm-up catalyst, including switching speed and voltage range, so you can see whether the sensors or the converter is the problem. Download Lewis.
Related codes
FAQ
- Is P0421 the same problem as P0420?
- Similar but not identical. P0420 covers the main converter. P0421 covers a smaller pre-cat. Not all vehicles have both; check your model.
- Will I fail emissions?
- Yes. Any catalyst efficiency code is typically an automatic failure in emissions testing states.
- Why does the pre-cat fail first?
- It sits right next to the exhaust manifold and runs hotter than the main converter, which shortens its life, especially on vehicles that do mostly short trips.