Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
We have a 2021 Chevrolet Equinox with a 1.5 turbo. It came in for a check engine light and no drivability complaints, scanned it and had a code of P2271(HO2S stuck rich sensor 2). We used propane and the O2 responded immediately and we created a vacuum leak and responded immediately to the lean condition. No other scan data looked out of range, long term and short term were around -4 to 4 and fluctuating as normal. e cleared the code and advised the customer to return the next week for a follow up. When the vehicle returned it has a code of P0420. We still se no signs of a bad O2 sensor or a bad Cat. Wave forms for both O2s look normal for a operating Cat and a good O2. Any advice would be greatly appreciated,
There's really only a few other checks you can do before it's cat time, IMO. One would be to check for exhaust leaks. Pressurizing the exhaust with a few PSI of air and using soapy water would be my suggestion.
Another check would be to run your VIN against SPS Calibration Information and see if there's any relevant updates available. I didn't find any TSB's for P0420 on these vehicles, so it's unlikely, but still worth checking. Let me know if you need help with this step.
No misfires? Oil, coolant consumption?
If that all checks out, you're done! Cat time. I suggest replacing both O2's along with the cat. Original equipment sensors only.
