code po121 tps

May not be TPS. Check wiring leading up to TPS.

First thing is to clean the throttle body. Carbon can cause the throttle to hang and the ECM thinks this is a TPS issue because it cannot close fully.

Next is to look at the data coming from the throttle control system, for that you need a scan tool with bi-directional control. With that on there you can wiggle the harness and see if it is a bad wire/loose connection or similar. You can also watch the system to look at the voltages it is seeing from the TPS.

I remember there were a lot of issues on those vehicles with wiring they were trying to push for recall. But Steve is right, clean the throttle body first and see..they may have given you a faulty sensor too. Good luck

Still are pushing for GM to do the replacement TBs and software updates but it is like pushing water with a rake.