What causes a P0421 code?

P0421 can be triggered by several issues — see the verified probable causes and step-by-step diagnostics shared by mechanics and owners on OBDIY.

Can I drive with a P0421 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with P0421 active.

Follow the step-by-step diagnostic on this page, then read the verified fixes contributed by other Heavy Truck owners — the answer marked "Verified Cause" worked for the original poster.

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OBD-II

P0421

Warm Up Catalyst Efficiency Below Threshold

What it means

Warm-up cat fault

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