throttle position sensor (P0121 code)

Just created a profile as i am running into some issues with our 2008 Yukon XL. I'm a fairly good mechanic, i think in where if i can get some direction i will tackle most any repair. Going to the dealership is typically my last resort. I am current working through some CEL issues for our emissions test before registering the vehicle in our newest state. Need so direction on how to reinstall a throttle position sensor as i have the P0121 code. Any guidance? BTW, I have already bought the parts and started the repair but the throttle response is not correct. It starts off fine with the key turn, but once you hit the accelerator it wants to stick around 2500 rpm and won't come back down.......
Nick

Have you put the old one back on to see if it does the same thing?

If it keeps with the new problem, then it is not the TPS...

Keep in mind I am old school... so some may not apply.

Try these:
Clean the MAF sensor (residue free electronics cleaner usually works, but they have special stuff for the MAF, I think they are the same thing actually)
It might be the IAC Idle air control valve sticking, (If I understand how they work, usually when you push on the throttle the IAC should stay at the same place, or go all the way open, this is so when you release the throttle it does not bottom out and cause the engine to stall... There might be some carbon built up causing it not to return down, or just enough that the power required to reduce the idle is not there, until you turn it off, at which point it resets itself...
I am not sure about the new systems, but the TPS itself is a sensor, and should not keep the throttle from going to ZEro or keep it from moving unless it was not installed correctly, it is a sensor, My 05 Rainier had a TPS fail, and I had to replace the entire throttle body...
I believe these trucks are controlled by wire, not a cable, so it might have to do with the throttle controller itself.

I have not looked on my 07 Tahoe, but on older cars you could see the mechanism moving and could tell if it was going back to Zero. Suggestion, 1) mark the Zero position on the throttle body, 2) then start the car and have someone make it stick at 2500 RPM, see if the throttle is not returning...

try tapping on the throttle body (not to break it) just to see if it goes back down when it gets stuck...

It could be a bad wire going to the IAC, not allowing it to move... Could be a bad connection going to the throttle control servo, It could be a bad ECM for that matter...

Good luck.