Welcome to CF !!!

Please use the below link to decipher and post the definitions to your found trouble codes. You will get far more advice when the members know what's possibly wrong - in this case, a throttle position sensor circuit fault series. I updated your profile to show what car we're working on (outside of the post text).

OBDII diagnostics. Trouble codes description for GM

OBDII/EOBD diagnostic code definition. More than 18000 codes for all autos. Trouble codes description for GM

obdii.pro

 

Sure, good idea:

P2138 - Accelerator Pedal Position (APP) Sensor 1-2 Correlation

P2123 - Accelerator Pedal Position (APP) Sensor 1 Circuit High Voltage

 

Update:

So I got the STS back from the dealership after 12 days. They claimed they fixed it...by replacing the Accelerator Pedal Assembly. As stated above I had already replaced the Accelerator Pedal assembly and an independent shop had also replaced the assembly. So 3 different Accelerator Pedal assembly were in the vehicle and all gave the same CEL. Huh, must not be that part. They said the new part was defective (the mechanic was "very confident in this") and replaced it for free under a "parts warranty". Very kind of them, instead of the $538 they originally quoted. As you can imagine i was pretty skeptical this fixed the problem. Of course less than 24 hours of having the vehicle back, the CEL is in again.

Back to square one. At this point it has to be a wiring issue or ECU issue. Somehow extra voltage is getting into the APP Sensor 1 circuit. What are everyone's thoughts on this possibly being an ECU issue? Plausible or no way?

 

Update:

So I got the STS back from the dealership after 12 days. They claimed they fixed it...by replacing the Accelerator Pedal Assembly. As stated above I had already replaced the Accelerator Pedal assembly and an independent shop had also replaced the assembly. So 3 different Accelerator Pedal assembly were in the vehicle and all gave the same CEL. Huh, must not be that part. They said the new part was defective (the mechanic was "very confident in this") and replaced it for free under a "parts warranty". Very kind of them, instead of the $538 they originally quoted. As you can imagine i was pretty skeptical this fixed the problem. Of course less than 24 hours of having the vehicle back, the CEL is in again.

Back to square one. At this point it has to be a wiring issue or ECU issue. Somehow extra voltage is getting into the APP Sensor 1 circuit. What are everyone's thoughts on this possibly being an ECU issue? Plausible or no way?

I've recently diagnosed a bad ECU on my 06 STS. I think there all starting to fail about right now. Maybe GM intended this as way to buy the latest V8 model. So you can confirm if the wires are intact to the the ECM by preforming a continuity test with a decent multimeter. I highly doubt your car has broken or rubbed through wire as that would be rare in the automotive world.

 

Figured I'd post the solution here in case anyone else has a similar issue. Took the STS to a different Cadillac dealership and they rewired the entire APP Circuit along with replacing all electrical connectors in the circuit. This appears to have solved the issue.

On a side note for anyone in the Denver area I highly recommend John Elway Cadillac. They were very helpful and professional and restored some faith in dealerships. They were actually interested in troubleshooting the problem instead of blindly following their code scanner. On the flipside the original dealership the car was at was Foundation Cadillac, and I can't say enough bad things about them. Do yourself a favor and avoid Foundation Cadillac at all costs.

 

Figured I'd post the solution here in case anyone else has a similar issue. Took the STS to a different Cadillac dealership and they rewired the entire APP Circuit along with replacing all electrical connectors in the circuit. This appears to have solved the issue.

See above, the problem was already fixed. It was the wiring or connectors somewhere in the APP circuit.

 

Try using a multimeter and see if all those wires for the throttle body have continuity to the ECM connector. When it comes to MIL complaint you have to start with the active codes and really understand and decipher what the computer is trying to tell you.

 

Well the dreaded P2138 - Accelerator Pedal Position (APP) Sensor 1-2 Correlation is back. It came back few months ago and is worse than ever. Seems like P2138 is a very common among all GM vehicles from 2005-2015. The cause appears to be water intrusion in the instrument panel to body harness connection according to TSB 07-06-04-019F. So insane to think this all could have been avoided by using a throttle cable. What the hell was wrong with a physical connection from the pedal to the throttle body?

Anyways has anyone found the exact source to this water intrusion and which connection has the water or corrosion on the STS 4.6? Not sure where the I/P to body harness is but I will start tearing connections apart until I find it.

 

When I had the same problem it was in the connector where the cable connects to the throttle body. I cleaned the connector pins and used a scribe to put a little more tension in them and reassembled it with some di-electric grease and so far the repair has lasted 19 years. When it first happened the car was new and it was the first real cold snap of the winter. New GM cars of various makes were lined up around the corner at the dealer, all with the same problem.

 

Thanks, yea I'll check it but its unlikely. Three years ago I replaced the TB and soldered in a new electrical connector to the TB and still had the P2138 code, telling me that wasn't the problem. But you never know with all these damn electrical connections everywhere.

Worst part is there are no drivability issues...the pedal makes the TB do exactly what I want it to do. Then this stupid code pops up out of the blue with no symptoms or signs of a problem and sends it to reduced power mode. Then I turn it off and back on. Back to normal until the code comes back and sends it to reduced power mode. Rinse and repeat. Honestly about to take this car to the junkyard because of it. Just a shame for a car that runs and drives so well.