I have a 2008 Impala LTZ with the dual-zone HVAC system (not automatic) - purchased new and never wrecked or damaged.
Last Fall, the a/c system had a very foul, musty odor when the system was turned on. At 19,003 miles, the dealer installed an "afterblow module" according to TSB #99-01-39-004C. The module part number was 12497910 (info. from the invoice). After installation, the module would momentarily turn on the blower while the car was parked without the engine running or key in the ignition, to dry out the evaporator.
Recently, while driving the car on the expressway with the a/c on for at least 15 minutes - set to outside air, bi-level setting and the lowest blower speed, the blower speed increased to the highest level for about 8 - 10 seconds, then returned to the lowest blower speed setting. This occurred a second time after the car was parked, restarted and driven again on the expressway. The outside temperature was in the upper 60's with very low humidity. It was warm enough outside to run the a/c on low while the car windows were up, but not hot enought to run the a/c on recirculating or higher fan speeds.
The passenger compartment air filter is not clogged - I changed it with a new Delco unit at 18,000 miles.
Today, I took the car to the dealer for the blower problem (now 23,300 miles - still under the new car warranty). The service advisor said there were no current or history codes and the blower module was "reset". He said that GM provides them with very little information concerning the operation of the blower module they installed according to the TSB. He was unable to tell me if the blower speed is supposed to increase to the highest level while driving, although set on the lowest blower speed. The blower speed increase while driving had not occurred during the five months since it had been installed, it began recently.
Any ideas, suggestions or information? It is annoying for the blower speed to increase to the highest level while driving, when it is set to the lowest level. Although the car has the dual-zone HVAC system, it is not an "automatic" system which turns the blower speed up or down. The control must be set manually. Also - if there is a problem, I would like to have it resolved - especially since the vehicle is still under the new car warranty. The dealer said to "bring it back" if the problem recurs, but I suspect they will not be able to diagnose the problem if I return.