P0421: Warm up catalyst efficiency below threshold (Bank 1)
Drive carefully. Get it checked within a week or two.
Can I drive with P0421?
Yes, but drive carefully and get it checked soon. Manifold warm-up catalyst is degrading; car runs normally but emissions fail.
What does P0421 mean?
P0421 stands for Warm Up Catalyst Efficiency Below Threshold (Bank 1).
Symptoms you may notice
- Check engine light stays steadily illuminated
- little noticeable loss in driving power or throttle response
- Rotten egg / sulfur odor from tailpipe in some cases
- Automatic failure during state emissions inspection
How much does it cost to fix?
Repair costs for code P0421 depend on the specific underlying root cause, ranging from simple DIY checks to component replacement. The table below lists the most common causes and rough price ranges in USD:
- Aged warm-up catalytic converter manifold
- Exhaust manifold crack before catalyst
- Downstream oxygen sensor aging
- Past misfires
What to do next
- Inspect exhaust manifold for hairline cracks.
- Check rear oxygen sensor operation.
- Replace the manifold catalytic converter.
Last updated: October 2026
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General guidance only, not a professional diagnosis. If the check-engine light is flashing, or the car is running rough, overheating or losing power, stop driving and get help.
About this code
Code P0421 refers to the close-coupled warm-up catalytic converter mounted directly to the engine exhaust manifold. Driving is safe, but replacement is required to pass smog.