View Full Version : Random de-rating

02-04-2021, 17:38
I just bought a 2006 Chevy C7500 with the 7.8L Duramax. It's my first Duramax, the engine sounds great, plenty of power (most of the time). At random times it will go into limp mode. Doesn't seem to matter if your on the highway or on country roads. It doesn't throw any codes either. I haven't taken it to the shop yet because I think it might be a loose connection? Someone said that it might be the knock sensor malfunctioning? I'm not sure where to start looking.
Any advice would be greatly appreciated.

02-04-2021, 21:51
Welcome aboard!

What transmission? Does the truck have updated smog equipment (DPF/DEF)? What brake system (air/hydraulic/hybrid)? Does it have ABS, and if so, is its lamp on while the engine is running? How many keys, fobs, etc. on the key chain (silly question, but you might be surprised)?

If it's limping, it must be generating codes, and the codes will be necessary for diagnosing all but the most obvious. Even if they self-correct and the SES lamp goes out, there will be codes stored in history a simple reader won't read. If it has a loose connection or a sensor, the code will give you a direction to look. It is unlikely a sensor, as those tend to be much more persistent, and the codes remain active for much longer periods. Perhaps you need a more capable reader/scanner, or a trip to a truck shop. Or maybe it's something more simple, like a plugged air or fuel filter, or a fuel leak, which can go unnoticed under many conditions, then show up at random times or specific conditions.