A Bad O2 Sensor.

[Nenous1]

Had a P0420 popup this past thursday and i was a bit surprised but not much, but it wasn't the catalytic converter, it was the downstream sensor..

Sidenote: Just before the engine light came on i noticed my gas milage dropped alot, is that common place'Or could something had caused that sensor to fail, something i'm over looking.

[Miami_SS]

I have the same issue too and had similar symptoms. It gave me an excuse to upgrade my stock downpipes to American Racing hi-flow downpipes with single cats (versus 2 for stock) and purchase 4 new O2 sensors from NAPA + add a Magana flow 2.5" cross-over pipe + 2.5" to 4" axle back exhaust. Sometimes a bad O2 sensor is a good excuse to mod/upgrade!

[Nenous1]

Originally Posted by Miami_SS

I have the same issue too and had similar symptoms. It gave me an excuse to upgrade my stock downpipes to American Racing hi-flow downpipes with single cats (versus 2 for stock) and purchase 4 new O2 sensors from NAPA + add a Magana flow 2.5" cross-over pipe + 2.5" to 4" axle back exhaust. Sometimes a bad O2 sensor is a good excuse to mod/upgrade!

The problem is' i just had a re-manufactured engine put in and i put new everything in and when a new O2 sensor goes bad for no reason, that's suspect to some sorta possible blow by went thru the exhaust system causing that failure.

Or it could be just a bad sensor.

[CamaroCracka]

A mileage drop would be associated with the front O2 sensor since that is input to the fuel injection system. The rear O2 only checks if the cat is working.