Having a problem with my 2000 C5 and a bit perplexed. I'll give you the entire info in short form here, hoping one of you can help me sort this out. Car has a FAST 90/90 on it with the pigtail to convert for the throttle body. BTW... all mods were done over a decade ago. I've owned the car since 2002.

ISSUE:
- Reduced Engine Power Mode
- Code P1120
- High idle RPM
- Throttle Body blade will not move based on gas pedal. But moves freely by hand

What I have tried to diagnose:
1. Tried installing a different (but same 90mm) throttle body that is known to be good
2. Used my pigtail adapter with the 2nd throttle body on another C5 to see if it works fine, and it did
3. Replaced the TAC Module with a known good one
4. Checked for any loose or dirty connections, but all seem to be ok

Using HPTuners:
- I checked the TPS % which was not moving based on gas pedal
- TPS % did correlate when I moved the throttle body blade by hand
- Tried reflashing the tune to make sure it isn't a logic problem

Using Tech II OBD2 Scanner
- Desired Throttle Position is not moving or registering
- Throttle Percentage does not move by gas pedal, but will move when I manually move the blade
- Voltage input does read fine when pedal is moved

The P1120 code refers to the throttle position sensor, but that seems to be working when I manually move the blade. The issue that I am having I THINK is coming from either the drive wire to the throttle body (which I cannot find any breaks in the wire yet) OR the APP sensor. Keep in mind that the same P1120 code comes on even when I hook it up to a completely new throttlebody and reset the codes. And I have not gotten the P1125 code that indicates that it is the APP sensor.


Any advice on diagnosis or where to go with this? I don't want to just keep buying parts that won't resolve the issue.