P0421 Code: Warm Up Catalyst Efficiency Below Threshold (Bank 1)
🟠 Moderate severity · drivable
Drivable, but the catalytic converter isn't working efficiently during warm-up and may eventually need replacing.
What P0421 means
The bank 1 catalytic converter isn't cleaning exhaust efficiently during the engine warm-up phase. This is similar to P0420 but specifically during warm-up, which often points to the warm-up (pre-catalytic) converter.
Symptoms
- Check engine light is on
- Reduced fuel economy
- Possible failed emissions test
- Often no noticeable driving symptoms
Common causes
- most common: Failing warm-up catalytic converter
- common: Faulty downstream oxygen sensor
- common: Exhaust leak near the sensor
How to fix it
| Fix | DIY? | Typical cost |
|---|---|---|
| Test the oxygen sensors and replace any that are faulty | Yes | $60–200 |
| Repair exhaust leaks | Mechanic | $50–200 |
| Replace the warm-up catalytic converter | Mechanic | $600–2000 |
Frequently asked questions
Can I drive with a P0421 code?
Yes, but the converter isn't working efficiently during warm-up. Get it diagnosed soon.
What is the most common cause of P0421?
A failing warm-up catalytic converter, though a bad oxygen sensor or exhaust leak can also trigger it.
How much does it cost to fix a P0421?
$60–$200 for an O2 sensor, or $600–$2,000 for a warm-up catalytic converter.
Related codes
P0420
P0420 Code: Catalyst System Efficiency Below Threshold (Bank 1)
P0430P0430 Code: Catalyst System Efficiency Below Threshold (Bank 2)
P0431P0431 Code: Catalyst 1 Efficiency Below Threshold (Bank 2)
Related guide: How to Reset a Check Engine Light →
Updated 2026-08. This page is informational and not a substitute for professional diagnosis.