2001 Mazda Protege LX 2.0L CEL Message of P0421

Maintenance/Repairs
1

Last year I posted my CEL problem of P0421 and replaced both upstream and downstream sensors. I passed smog because I was allowed one error message and that message was Cat not ready. I had cleared the CEL light and drove long enough before inspection to reset all but the Cat not ready message and no CEL.

Last Friday I had to clean my throttle body butterfly valve and interior housing and I also had to replace the air inlet hose that connects the air cleaner/MAF sensor to the throttle body because the hose had some splits allowing air in. I disconnected the battery before doing any work.

My new question:

Could air entering freely through the splits in the inlet hose, thus bypassing the MAF sensor, and being delivered directly to the throttle body, cause an error message of P0421?

2

Probably not. I’m pretty sure that error code is generated by the sensor downstream of the converter. Vacuum leak types of issues such as yours would show up in the upstream sensor readings. You’re basically left with faulty wiring for the rear O2 sensor, exhaust system leak(s) (don’t assume you have to hear or smell them), or an actual bad converter.