194k on the car
I've had it for 8 years and I don't recall ever replacing the O2 sensors. So perhaps it's time anyway.

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The rear sensor looks fine. If replacing, do the front sensor. Historically, the ACDelco have been private labeled denso's. Check rockauto.com for the AC Delco part number for the OEM sensor, then you should be able to match it up to the denso sensor with the correct part interchange number. Identical sensor for $15 less. Amazon sometimes undercuts RockAuto but also sometimes the sellers on Amazon gouge to hell on Saturn parts. I'm not sure about the OEM manufacturer for the front O2 so I would suggest doing a bit of research and nor just believing what I just typed.

Oh and don't even consider the universals.

 

The wiring harness for that O2 sensor got tucked up under the heat shield. I'm sure I did that with all the work I was doing, replacing the engine and all.
Anyway, the plugs were melted together. Nice.

looks like I got some rewiring to do...

 

I'm sure pigtail is available for that...somwhere

 

New O2. Rewired.
not much change.

 

Did you clear the codes and drive an OBD 2 drive cycle? Drive it long enough to run all the monitors? The system has to relearn and this is the best way to speed up the process.

 

I cleared the codes, but the cycle is not complete yet. It takes forever with this car. Still driving, but the P0172 code came up after 5 miles.
but I will keep the faith...
Drive drive drive

 

Which monitors have not run? Usually the evap is the last to run since it has such strict requirements to even run. Highway speed and the fuel has to be between 1/4 and 3/4 full. After an 8 hour cold soak usually.

 

Does your car have a Mass air flow sensor? Have you checked fuel pressure yet? I would imagine you have a pwm fuel pump with no regulator or return. Something is making it rich, restricted intake or exhaust. How does the exhaust sound? Can you hear it fire or is it more like air noise? A restricted cat will muffle the cylinder firing noise.

 

Does your engine have EGR, probably not with vct?