You may not believe this, but being able to trigger the failure pretty much on demand is a GOOD THING. When you can't get the malfunction to happen, it's a REAL head scratcher. If YOU are going to attempt to fix this, go to GMUPFITTER.COM and get the electrical manual in the body builder manual tab for your truck, find the a/c controls and see what you can find for air conditioning. There will be a lot. My manual not only has the diesel truck info, but gas truck also, so watch what pages you are looking at. The find function works very well, so use it. It could be that the problem is something as simple as a ground for the HVAC fan that has too much resistance to carry the intended load, or a problem within the LAN. LAN problems can be hard to pin down, so you may have to find other devices on the same circuit as your a/c controls, detach them, re-test to see if the problem has gone away, repeat with another different module, repeat with another different module, repeat with another different module, etc until you find the problem module. Personally, I would start with everything in the truck turned OFF, particularly anything aftermarket like engine monitors, radios, anything plugged into the cig lighter, radar detector, etc, doors closed, toggle your a/c control to create the problem, and go from there. It could be a combination of things, like radio on plus fan on.
See below from my truck's info about LAN communication:.
GMLAN LOW-SPEED CIRCUIT DESCRIPTION
The Data Link Connector (DLC) allows a scan tool to communicate with the low speed GMLAN serial data circuit. The serial data is transmitted
over a single wire to the appropriate control modules. Under normal vehicle operating conditions, the speed of the buss is 33.33 Kb/s. This
protocol produces a simple pulse train sent out over the GMLAN low speed serial data bus. When a module pulls the buss high, 5 volts, this
creates a logic state of 0 on the buss. When the buss is pulled low, 0 volts, it is translated as a logic state of 1. To wake the control modules
connected to the GMLAN low speed serial data buss, a wake up signal is sent out over the buss. Modules connected to the GMLAN low speed
buss can be part of a virtual network as described in GMLAN High Speed Circuit Description above. The modules on the GMLAN low speed
serial data buss are connected to the buss using several splice or "star" connectors separating groups of modules. The following list states the
splices and modules connected to the low speed serial data circuits:
• Data Link Connector (DLC), connected only to the Instrument Panel (I/P) splice.
• Amplifier (Amp)
• Rear Seat Audio (RSA) .
• Vehicle Communication Interface Module (VCIM)
• Digital Radio Receiver (DRR)
• Inside Rearview Mirror Module (ISRVM), connected through the mid I/P fuse block
• Instrument Panel Cluster (IPC)
• Body Control Module (BCM)
• Theft Deterrent Module (TDM)
• Heater Ventilation and Air Conditioning (HVAC)
• Driver Door Switch (DDS), connected through the left I/P fuse block
• Passenger Door Switch (PDS), connected through the right I/P fuse block
• Articulating Running Board Module (ARBM)
• Ultrasonic Park Assist (UPA)
• Memory Seat Module (MSM)
• Liftgate Module (LGM)
• Passenger Presence System (PPS)
• Inflatable restraint vehicle Rollover Sensor (ROS)
• Inflatable restraint Sensing and Diagnostic Module (SDM)
Serial Data Reference
The scan tool communicates over the various busses on the vehicle. When a scan tool is installed on a vehicle, the scan tool will try to
communicate with every module that could be optioned into the vehicle. If an option is not installed on the vehicle, the scan tool will display No
COMM for that options specific control module.
In order to avert misdiagnoses of No Communication with a specific module, refer to Data Link References for a list of modules, the busses
they communicate with, and the Regular Production Option (RPO) codes for a specific module.
2008 Silverado LT, CCLB, Edge Insight CTS
2009 Arctic Fox 29-5T, Silver Fox Edition, Onan 3600 LPG generator