2001 Chevrolet Impala 3.8L P0420 Fault: Converter Below Threshold Despite CAT Ready Status
Hi, I'm experiencing a check engine light on my 2001 Chevrolet Impala with the 3.8L engine, showing a DTC P0420 indicating 'Catalyst System Efficiency Below Threshold'. The vehicle has cleared all I/M readiness monitors except for EVAP. The catalytic converter status in the OBD-II system displays as 'OK' and marked ready, yet the check engine light remains on. Live data shows the downstream oxygen sensor fluctuating between 0.8 and 0.105 volts. Given this behavior, is it possible or likely that the downstream oxygen sensor is faulty rather than a failing catalytic converter? The fact that the converter appears ready in the system but still triggers P0420 suggests an inconsistency. Could this be due to a defective sensor causing misreadings instead of actual converter failure?
The catalytic converter monitor shows complete status, yet the vehicle is still throwing a P0420 code — this sounds like a potential software glitch in the OBD-II reader. I've personally never seen an oxygen sensor reading resolve a P0420 issue. If the downstream oxygen sensor exhibits significant oscillation (like the upstream one), it's highly likely that the sensor is faulty, and the catalytic converter itself is not the root cause — this scenario occurs in approximately 95% of such cases.