P0422 indicates that the downstream oxygen sensor has detected that the efficiency of the catalytic converter on bank 1, catalyst 2 (the rear converter), is below the manufacturer’s threshold. The ECM interprets this as the converter not adequately reducing emissions.
Age, overheating, or contamination can cause the converter’s catalyst material to lose effectiveness, triggering the code.
A leak before the sensor can introduce extra oxygen, making the sensor read a lean condition and falsely indicate low converter efficiency.
A sensor that is slow, stuck, or out of calibration can misread the oxygen content and cause the ECM to think the converter is under‑performing.
Consistently running too rich or experiencing misfires can overheat or poison the catalyst, reducing its efficiency.
Restrictions or cracks can cause abnormal exhaust gas temperatures that degrade the catalyst’s performance.
A malfunctioning control module can incorrectly interpret sensor data, but this is an uncommon cause for P0422.
No. The PCM is rarely the root cause of P0422. Replacing it without confirming a mechanical fault usually does not resolve the issue and adds unnecessary cost.
Only if the upstream sensor is proven faulty. The code specifically monitors the downstream sensor’s reading relative to the converter’s performance, so the downstream sensor and the converter itself are the primary suspects.
It can happen if the converter was damaged during manufacturing or if there is an exhaust leak. In a new vehicle, the first step is to verify there are no leaks and that the downstream sensor is functioning correctly before considering a converter replacement.