P0422: OBD-II code P0422 flags that the main catalyst on bank 1 has fallen below efficiency threshold — emissions clean-up is degraded despite good O2 sensor operation.

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 vehicles.
• Often accompanies P0420 as a companion code as the catalyst pack ages beyond usable life.

Common causes

• A worn or damaged main catalytic converter on bank 1 that has lost oxygen storage capacity — the most common cause.
• An underlying misfire, rich condition, or oil burning issue that damaged the catalyst — always fix root cause first.
• A failing downstream O2 sensor reporting inaccurate switching, mimicking a bad catalyst.

What to do?

Address any misfire, fuel-trim, or O2 sensor codes before condemning the catalyst. Compare upstream vs downstream O2 waveforms during closed-loop operation — downstream should stay steady. Replace the main cat with an OE-spec part if genuinely degraded.