P0421

What the code means

P0421 flags reduced efficiency of the warm-up catalytic converter on bank 1. This converter sits closer to the engine and is designed to cut emissions quickly after a cold start, while the main catalytic converter is still warming up to operating temperature.

The control module compares the warm-up phase conversion efficiency against a set threshold. If actual performance falls below the threshold, the code sets. Causes include a failed catalytic converter, an exhaust leak upstream of or at the sensors, or a faulty oxygen sensor skewing the readings.

Symptoms

• Check Engine light is on.
• Higher emissions during cold starts.
• Possible exhaust smell.

Diagnosis and repair

1. Read the fault codes and check oxygen sensor data — the readings let you tell real catalyst degradation from a sensor sending a bad signal.
2. Check the exhaust system for leaks: air getting in upstream of the oxygen sensors skews the readings and triggers a false code.
3. If the sensors and system sealing are good, replace the warm-up catalytic converter.
4. Check the cold-start system: if it isn't working right, it affects the conditions under which catalyst efficiency is evaluated.

Secondary problems

• Higher emissions during cold starts.
• Possible clogging of the main catalytic converter from unburned combustion products — don't ignore the code, or you risk losing the second converter too.