2011 GMC Sierra defrost not working properly

This problem is with all pickup truck-based GMC vehicles in the 1500-3500 series from 2010 to present. Margaret in our Ask a Mechanic forums brought this issue up with her 2011 GMC.

Big Block 409 quickly answered by posting a service bulletin (TSB 06-01-38-003), which deals with several air conditioning and defrost problems, including this one.

This first began to surface in early 2010 models in cold, humid climates (Great Lakes, seaboards, Canada). Originally, it was thought to be a problem with the A/C compressor, since it does not work at temperatures below 0 to -1 F. A software update, however, did not remedy the issue for everyone and the problem was happening in trucks that were not in severe cold.

Further analysis found that the defrost would not disengage the air recirculation in the cab, even if the driver pressed the button to shut it off. That requires a physical repair, which was simple, but required the trucks be brought to the dealership. The TSB covers that issue plus several other safety concerns with the defrost as well as climate control issues with the air conditioning.

The reason for the defrost problem is that air recirculating carries the same amount of humidity as before and so it only picks up more as it flows around the cabin and the occupants' breathing. Humidity causes fog and ice on cold windows. Normally, when this is happening, the vehicle or driver will turn on the A/C to dehumidify the air or turn off air recirculation to bring in outside air (that can be heated by the truck) to lower the amount of humidity and thus boost the performance of the defrost. Since neither was possible in these GMC trucks, only opening a window would work - leaving occupants uncomfortable in cold and/or wet weather.

The fix can be done at any authorized GMC dealer and will cost the owner nothing.