Original Resonator rusted-out. Too much noise made bringing Monte to Muffler Shop for fix.

Resonator and long section of pipe going back toward Muffler replaced.

Check Engine Light: ON
Code reads O-2 Sensor at fault.

Related? Or, ... Coincidence?

90,000 Miles / Heatwave / No A/C / Reg Gas w/ 10% Ethanol

 

Sounds more like coincident to me.

 

This might not be the problem, but Id get rid of the 10% ethenol. I think thsi is what Ive heard. Anybody feel free to correct me on this, cause I dont wanna be wrong.

 

The ethanol would seems to throw the P0440 Code for Fuel evap. I know my monte is real sensitive if I go to certain gas stations it throws the code till the fuel cycles through to what I normally put in

 

Which O2, front or rear?

Regardless, the muffler shop shouldn't have touched anything up there- the closest O2 is behind the cat, and that was way ahead of where they would've been working.

If it is the rear, maybe look under there to make sure they didn't unplug it, or damage the wiring or anything stupid- but it sounds like you just need a new O2 sensor.

Also, I don't see the ethanol causing a problem, I've run a lot of that gas through my Impala with no check engine lights- seems like the bulk of the stations out here have it.