ChrisMills

River, Don't want to jump the gun but this seems mostly solved. The problem at hand was that it was stuck in reduced engine power with no solution in sight. I really appreciate your help. It got my truck restoration project unstuck and moving forward again. Was figuring a 6 - 12 month time frame for the project when i bought the truck last January. ( It's a patient process. ) The very front of the truck needs some work. Was planning on replacing the front bumper & power steering cooler anyway so might as well pull the grill and open the front right up. You know ... give it all a good degreasing, inspect the entire wiring harness for damage, remake all of the ground contacts and so on. ( Whatever it needs to bring it up to spec.) It's a good bit more work and expense but it's worth it. The work holds if it's done right the first time. I might not have to go back in there for as long as i own the truck. Again ... many thanks for your help and guidance.

Pulled headlight assembly and inspected wiring in that area. No visible damage. Broke two electrical harness connectors and no sign of corrosion. (Side note: Discovered that the power steering cooler has a leak at the top. That area is a bit of an oily mess.) Pulled bulb: The truck started and shut down normally. Throttle response a bit sluggish but it was there. No power to dash cluster. Re-installed bulb: Dash cluster out, but the truck appeared to be in reduced power mode. No throttle response and truck shut down abnormally.

Sounds good. Pulling the Lo HDLP-R fuse had no positive effect. Pulling the Lo HDLP-L fuse had a positive effect. The left ( driver side) headlight & dash cluster were knocked out but the truck started and shut down normally. There was throttle response so it's probably safe to assume that the truck wasn't in reduced engine power mode.

Ok .. i went to the under hood relay center and pulled all the non-essentials one at a time. When i removed the HDLP-LOW relay the truck started up and shut off normally. No reduced engine power message and the throttle works just fine. This is the first sign of progress in months.

2004 GMC Sierra 2500HD 6.0l 4wd auto trans Awhile back the truck went into reduced engine power mode on startup one morning. I turned the key to the off position and removed it. The engine kept running and the dashboard lights flashed. Discovered that i can stall the engine with key in run position by turning on the high beams. The driver side headlight is dim with DRL running. Got trouble code U0107 Normally this would seem pretty straightforward. It's saying that there's a problem with the throttle control system somewhere. The thing is that the issue of the engine not shutting off with the key removed complicates things. It suggests that something else is going on. I originally posted this question in the wrong section as i'm a newbie here. There's a bit more background in that thread. https://www.gm-trucks.com/forums/topic/226118-stubborn-problem/ Anyway ... been going round and round with this one for awhile now. Have researched the heck out of it and tried all kinds of things but not making any progress. Maybe i'm too close to it and missing the obvious ? Maybe somebody else has seen this before ? Any suggestions on what else i might try ?