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To get your signature filled out, go to your User CP options, and select signature. Basic info is very helpful (use other's as an example), but your truck isn't as common around here, so more detail is better. If you can't figure it out, please email or PM (Private Message) me or a moderator, like Mark. I'll move the thread to the Medium Duty Forum. The Duramax Forum is intended for the 6.6L Duramax.

Now, as far as your problem goes, it doesn't sound really unique to the 7.8L. Most problems are Diesel engine related, and can apply to any Diesel engine. The Dealer can diagnose problems w/o codes, but many are too lazy (for lack of a better term) to do real technician work anymore. You need to find a good Medium/Heavy Duty GM/GMC authorized dealership to take care of you. Very few issues that cause poor running will cause no codes. Fuel quality is one of them. If you are not using an anti-gel additive, try it. It could be simple as that. Also, check the air filter. If it's getting any moisture in it (like an airbox leak), it will freeze up and block airflow. If it were a sensor problem, you'd certainly have DTC's (SES lamp) stored in the PCM memory, which will still be there after the SES lamp goes out. And, it could be a transmission issue, which will cause low power mode and the SES lamp, but needs to be diagnosed separately from the engine.

We'd like to help you, and will given the chance. At the very least, someone else will be helped by the experience. Please keep us updated often with any new information.