P0421: Catalyst 1 Efficiency Below Threshold Bank 1
What to do
Driveable, will fail an emissions test. The same caution as P0420 applies — establish that the converter is genuinely at fault, and find what damaged it, before replacing it.
What this code means
The close-coupled warm-up converter on bank 1 is underperforming. This is the small converter mounted hard against the exhaust manifold, positioned there deliberately so it reaches operating temperature within seconds of a cold start — most of a journey's total emissions are produced before the main converter is hot, which is the problem the warm-up unit exists to solve.
Its position is also what kills it. It runs hotter than the main converter and takes the full force of anything the engine sends downstream, so a misfire or a rich condition destroys it first. Diagnose it exactly as P0420: check for exhaust leaks, test the sensor that reported the fault, and read the Mode 06 monitor value before condemning the part.
Likely causes
Start here
- Failed catalytic converter · specialist
- Exhaust leak before the sensor · moderate
- Aged or lazy downstream oxygen sensor · easy
Less common
- Misfire dumping unburnt fuel into the converter · moderate
- Prolonged rich running damaging the converter · moderate
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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