The Lemon IndexThe small warm-up catalytic converter on bank 1 isn't working as well as it should.
Drive on, get it checked. You can usually keep driving. Have the code read at your next service or within a few weeks.
Worn converter. Then: faulty O2 sensor; exhaust leak.
Owners named P0421 in 10 complaints to the U.S. government, most often on the Mitsubishi Lancer.
Cars owners report it on
Complaints that name P0421, by model. Raw counts, not a failure rate: popular models get more complaints.
The details
What P0421 means
The small warm-up catalytic converter on bank 1 isn't working as well as it should.
Can I drive with P0421?
Drive on, get it checked. You can usually keep driving. Have the code read at your next service or within a few weeks.
30% of the owner complaints that name P0421 also say the car shakes, runs rough or misfires. A complaint can describe more than one problem, so this doesn't prove the code caused it.
Common causes of P0421
- Worn converter
- Faulty O2 sensor
- Exhaust leak
A shop should confirm the cause with a scan and tests before replacing parts.
What owners describe
- Shakes, runs rough or misfires: 30% of complaints
- Uses or leaks oil: 20% of complaints
Share of complaints naming the code whose text also mentions each problem.
Related codes
P0420 P0418 P0430 P0411 P0410 P0406 P0405 P0404
Quick answers
What does P0421 mean?
Warm Up Catalyst Efficiency Below Threshold (Bank 1). The small warm-up catalytic converter on bank 1 isn't working as well as it should.
Can I drive with P0421?
Drive on, get it checked. You can usually keep driving. Have the code read at your next service or within a few weeks.
What causes P0421?
Worn converter; Faulty O2 sensor; Exhaust leak.
Counts: NHTSA owner complaints whose text names P0421. A complaint is an owner's report, not a diagnosis. Meanings: the standard SAE J2012 definitions in our own words; manufacturer codes only where verified for that make. General information, not repair advice for your car.