P0421: The warm-up catalytic converter is not cleaning exhaust well
Warm Up Catalyst Efficiency Below Threshold (Bank 1)
The bank 1 catalytic converter is slow to start working after a cold start. Get it checked within a week or two. If the light flashes or the engine shakes, that is a misfire: stop driving.
Stop driving and get help if
- The check engine light is flashing
- The temperature gauge runs high, or you see steam (overheating)
- The car loses power, hesitates badly or stalls
- You smell burning or see smoke
- A brake warning light comes on, or the brakes feel different
Pull over as soon as you can, turn the engine off, and call a tow or roadside help.
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 the fix could involve
From the cheaper fix to the bigger one. Prices depend on your car, your area and the shop, so ask for a written quote that names the part and the labor before you approve it.
- Oxygen sensor replacement
- Exhaust leak repair
- Warm-up catalytic converter replacement
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 reads P0421 alongside any oxygen sensor codes and shows the live upstream oxygen sensor reading and fuel trims, so you can see whether the engine is running clean before you blame the converter. Get Lewis on the App Store or Google Play.
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.
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