P0420 / P0430 - Catalyst System Efficiency Below Threshold

The Technical Definition: The oxygen sensors are reporting that the catalytic converter is not cleaning the exhaust gases efficiently enough. P0420 is Bank 1, P0430 is Bank 2.

Common Symptoms: Check engine light is on. Possible sluggish acceleration or a faint sulfur (rotten egg) smell from the exhaust.

Typical Fix: Sometimes this can be triggered by a faulty downstream O2 sensor, which is a cheap fix. However, if the sensors are good, it usually means the catalytic converter itself is failing and needs to be replaced. Exhaust leaks upstream of the sensors can also trigger this.