OMG!! I've been fighting this for 2 years. Here's the history...
2002 C5 coupe - no mods, 31k miles. Previous owner had the AP replaced and grounds cleaned due to Reduced Engine Power / Service Traction Control ($700). I had it a little while and errors started showing up again. Varied somewhat but always related to the APP/TAC system. There are some posts related to the TAC on this forum where I gave some of that history.
I recently troubleshot (or so I thought) it to the TAC module. I took the PC board out of the plastic case, hooked it up, started the car and at will I could cause the car to stall with error codes similar to what I was getting just by barely flexing the board. $450 for a reman TAC module from PSI. Put it in and everything is great for about 500 miles. Then a couple days ago while I had a 200lb passenger in passenger seat - I get REP/STC again ! It's bad. It's repeatable (4 times just driving <10 miles. The difference from before is it's always the same codes - P1125 - APP system; P1286 - APP Sensor 3 Circuit Performance; C1278 - TCS temp. inhibited by PCM. I'm thinking this issue is the cable or connectors. If someone wants to tell be otherwise, I'm all ears.

My concern is I really can't see how the cable is routed, especially through the firewall. Does anyone have pics of where the cable goes from end to end (from the AP, through the cabin to the firewall - and from the firewall to the TAC module). I'd like to know what I have to remove to get to the cable, what harnesses I have to cut open to separate the TAC harness from the rest, etc. . If I can avoid the replacement, tell me what to look for and how to fix it. I have the factory service manuals so if you can, refer to them and I can look it up. :confused2

I'm a sad, frustrated, twice bitten, fleabag of a vette owner who has been humbled by this electronic beast of a car. I need some divine forum guidance

 

I didn't get much response but Bill Curlee did send me a diagram. From that I looked for drawings in the factory service manual and I'm posting drawings of where I think the Accelerator Pedal (AP)/ Throttle Actuator Control (TAC) module cable is routed.

First, I think it's in a wire bundle on the engine compartement passenger side by the battery and the A/C lines. It looks as though this is where most (all?) wire bundles feed through the firewall and inside the dash area.



The entrance from the inside of the car is in the passenger foot well where the BCM and fuse/relay block are located. The wire bundles look to be under/behind those items so they may need to be moved/removed to get to the firewall where the wire bundles comes through.

To the left of that, it appears there is a frame rail that supports the dash where some of the wire bundles go to. I'm guessing the AP/TAC cable follows this route.



Here's a drawing that seems to show that the frame rail traverses all the way across the dash to the driver's side of the car.



It looks as though some of the wire bundles exit the dash frame rail on the driver's side of the radio/HVAC controls.



Finally the AP/TAC cable attaches to the Pedal. You can see that it's a part of a larger wire bundle.



My plan is to see if I can find the AP/TAC cable and follow it through, looking for damaged wires/connectors. I'll try to tighten and clean the connections themselves first and look for easy and obvious places where I can inspect the wires. If that doesn't work, I'll be getting a new cable and route through the path shown (I'll leave the old cable in there as it's probably extensively bundled with other wires).

This time I'll take pics of how to get into the TAC module area (seems like some folks were asking and there used to be pics but I guess the links are gone) and will post them later.

 

OP, Did you find your problem? I just started chasing the reduced power problem with a constant current 1220 and 1221 codes on the DIC. Car won't even start. I did replace the TPS with no change.