P0420 Fault on 2002 Chevrolet Malibu - Catalyst System Efficiency Below Threshold
I received a P0420 diagnostic trouble code back in September 2012, and had the catalytic converter and downstream oxygen sensor replaced at a repair shop. Recently, the check engine light has returned with the same P0420 code. Given that my vehicle is now at 121,000 miles and I've noticed no improvement in fuel efficiency, I'm wondering if the front oxygen sensor may have failed or whether the previously installed catalytic converter and rear oxygen sensor are faulty. The P0420 code indicates a catalyst system efficiency below threshold, which suggests the exhaust conversion process is not functioning properly.
The P0420 code stands for 'Catalyst System Efficiency Below Threshold (Bank 1)'. While an oxygen sensor reading can sometimes produce a false positive, it's unlikely that the front O2 sensor is at fault given that it was already replaced. The issue is more likely related to the catalytic converter or the downstream oxygen sensor performance. I recommend returning to the same shop that performed the previous replacement and requesting a full catalyst system efficiency test to confirm the root cause.