Hey guys I have a 2004 GMC Yukon Denali that has the dreaded Reduce Engine Power message on along with the CEL T/C light.

As I carry a obd2 scanner i checked the codes and have 4 codes,
P2135 P0120 P0220 P1516.

I am lost here on what to check. I switched throttle bodies from a silverado and that did not fix the issue. What has me more stumped is that when I erase the codes with the engine not running as soon as the scanner erases them the codes pop up right that instant.

Seems like its not reading whatever it has to read before the engine turns over.

 

I would suggest you inspect the wire harness that runs to the throttle body....
if the wires or connector are damaged its going to flag it pretty quickly as not seeing those items... might need to check each signal pin for signal/continuity

 

Sounds like I should re check them again. I should look for a diagram of the connector to see which ones should have continuity and which ones are ground.

I also did the wiggle test but they all seem to have no free play or feel loose.