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Discussion Starter · #1 ·
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.
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