P0421: OBD-II code P0421 means the warm-up (close-coupled) catalyst on bank 1 has fallen below efficiency threshold during cold-start emissions monitoring.

Symptoms

• Check engine light is on; the vehicle drives normally but will fail an OBD-II emissions test.
• Fuel economy may drop slightly, and a sulphur or rotten-egg smell can appear from the exhaust on some models.
• The code often accompanies P0420 as the catalyst pack ages and both main and warm-up cat efficiencies fall.

Common causes

• A worn or degraded warm-up catalyst that no longer stores and releases oxygen efficiently on cold start.
• An underlying misfire or rich condition that has damaged the catalyst — fix the root cause first.
• A failing downstream oxygen sensor reporting inaccurate switching behaviour to the PCM.

What to do?

Address any misfire, fuel-trim, or O2 sensor codes before condemning the catalyst. Compare upstream vs downstream O2 sensor waveforms during warm-up — the downstream should stay steady, not switch. Replace the warm-up cat if genuinely degraded, using an OE-spec part to prevent the code from returning.