If its a California emissions truck it's probably got a burned out glow plug. If it is California emissions there is a special policy on them that covers them for 10 years or 200,000 miles. If its a federal emissions truck it's is more than likely a glow plug control modual relay.
Troubleshooting for p0380 from GM diagnostics... follow it from # 1 until you get to your answer or locate the problem and DO NOT assume anything or skip a step. If you do you will never fix it. Forget replacing the relay as suggested, go through the steps one by one
General Motors code p0380 means you have a wiring problem, the glow plug relay has failed, or the ecm has failed. First you can replace the relay to see if that takes care of the problem if not you will need to do some wiring tests. Here is a diagram and I circled the two wires that are in question. First you can turn the car on and with a test light check the pink/yellow wire going to the relay for short to power. So put the test light lead on battery negative and with the key on check the pink wire, if the light lights up this means there is a short to power and you will need to run a new wire. After this you can put the test light lead on battery power and then touch the test light to the wire and it should light up, this will indicate it has a good ground. By now we verified the pink/yellow wire is good so we need to move on to the yellow which goes between the relay and ecm put one lead on battery power and the test light on the Yellow wire, if the light lights up this means there is a short to ground and you will need to run a new wire between the ecm and relay. If not the next step is to unplug the connector at the ecm and do a resistance test between the ecm and relay on the yellow wire if resistance is high replace the wire and also make sue the connections are clean. If everything looks good up to this point the only possible cause is the ecm.
Trouble shooting sequence, start at # 1 and confirm findings step by step.
#1 cause of that code that I've seen is a glow plug. Just check the ohms on each glow plug and find witch ones have high resistance or are ol on a multi meter. I have done hundreds of glow plugs for this code. Let me know if you need help as I am not to far from you and I am a duramax tech.
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