OBD-II Code · Emissions
P0421
Warm-Up Catalyst Efficiency Below Threshold (Bank 1)
medium severitySafe to drive$100-$2,000
Pre-cat efficiency low on bank 1.
Common symptoms
- CEL
- Failed emissions
Likely causes
- Aging cat
- Exhaust leak
- Misfire upstream
Where to start
- Try the cheapest cause first. Start by checking: aging cat.
- Cost & scope. $100-$2,000
- If the code returns after the fix: escalate to a shop or scanner with live-data and freeze-frame. A code that re-sets means the underlying fault is still there.
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