Check Engine Code
P0421
Warm-Up Catalyst Efficiency Below Threshold (Bank 1)
What P0421 means
Some vehicles have a small 'warm-up' catalyst right at the exhaust manifold to start cleaning emissions before the main catalyst lights off. P0421 means that warm-up cat is degraded.
Symptoms
- CEL
- Slight emissions/economy impact
- Test fail
Likely causes (most common first)
- Aged warm-up catalyst
- Misfire damage
- Oil/coolant contamination
- Aged O2 sensors making it look worse than it is
Can I keep driving?
Medium — drivable but fix soon
Typical repair cost
DIY fixes can start near $100 (parts only). Shop repairs for P0421 typically run up to $1500 depending on the underlying cause and vehicle. Start with the most common cause — Aged warm-up catalyst — before throwing parts at it.
Recommended parts & tools for P0421
Our buyer’s guides cover the tools and parts that pair with AXLY:
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