Replace your exhaust manifold bolts stat, and get the new ones online or from O'Reilly (I think $15.99 or less for all 12) make sure grade 10.9 so they won't break off in your head. If you haven't fixed this yet don't let the dealer do the job or if you do provide the bolts as the ones they use will break shortly. I was going to get the bolts from the dealer because I wanted them quick and everyone else was sold out, but they were $6.99 each...
Problems for specific GMC Sierra 2500 HD years:
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Newest reported 2004 GMC Sierra 2500 HD problems
2001-2007 GMC/Chevy... Transfer cases have a clip that rubs through the soft magnesium housing near the pump and ALL TC fluid is blown out during driving. No leak spot, no way to know until catastrophic failure around 90K-150K miles (Mine went out at 140K). If you havn't already replaced your TC, get an upgrade kit... Buyer beware, NEVER purchase an effected vehicle without checking the TC for quick-weld. There is much info/pictures online...
Illumination of the Check Engine Light can be caused by a loose or worn gas cap.
The keyless entry system may stop working. Our technicians tell us the key fobs and keyless entry receiver may need to be replaced to correct this fault.
The 4WD transfer case encoder motor position sensor or the selector switch may fail causing the service 4WD message to be displayed. Fault code(s) stored in the transfer case control module will be of assistance in determining the exact fault causing the message to be displayed.
One or more heater & AC (HVAC) temperature and/or air delivery mode door actuator may fail, resulting in improper temperature or air delivery. If this occurs, fault code(s) should be stored in the HVAC control module which should assist in diagnoses.
One or both secondary air injection check valves may become restricted causing poor air flow resulting in illumination of the Check Engine Light. It is often recommend to replace both check valves even if a fault code is set for only one bank of cylinders. Replacing only one valve may result in a return trip to the repair shop.
A clicking noise may be noted from the turn signal flasher after the turn signals are switched off. This is commonly cause by a faulty turn signal switch which will require replacement.
The daytime running lights are prone to burning out. Our technicians tell us that the light sockets may show signs of heat damage but usually are OK. Always be sure to use the correct replacement bulb for the best life expectancy. The new bulb recommendation for 2001-2003 models is #4114K (trade number).
Units with 4 wheel disk brakes may develop at situation where the park brake does not hold as it should. Our technicians tell us this can be caused by worn park brake shoes located inside the drum section of the rear brake disks. The worn shoes should be replaced and properly adjusted to correct this issue.
A poor electrical ground under the battery tray may cause the ABS warning light to illuminate. Our technicians tell us that cleaning and properly securing this ground and also the main ABS ground on the frame rail below the drivers door may be necessary.
The fuel pump may fail causing the engine to stall and not restart.
It is common for the fuel level sensor in the fuel tank read the fuel level incorrectly. The sensor must be replaced to correct the condition. Our technicians recommend having the complete fuel pump module replaced on high mileage vehicles.