Rear 02 sensor error

[cvc]

Verify no leaks on you exhaust and the GM single wire sensor is under $20 throw one in then check your live data warmed up with the seat off do a wiggle test on the harness and pay close attention to any spikes in O2 data. I have several B bodies and keep a sensor on the shelf that fits them and my Buells the box is getting old they last so long but still cheap insurance to put in a new one first.

Then follow Shaughn’s advice and check fuel delivery. That is more time consuming and if you get lucky the sensor will fix it.