Many new cars and trucks now have automatic temperature control (ATC) systems that not only regulate cooling but also heating for year-round passenger comfort. Most of these systems have their own computer that may be built into the control panel head, located elsewhere or integrated into the body control module. One thing's for sure: the more sophisticated the system is, the more complex are its control electronics and operating logic - which increases the odds of something going wrong.

Simple manual temperature controls are being replaced with digital push button settings and computer logic. Today's new car buyers want separate controls for the driver and front seat passenger, and triple-zone rear A/C in minivans and SUVS with a separate control head in the rear to keep the kiddies or mother-in-law comfortable.

ATC systems require a complex array of internal and external sensors including ambient air temperature sensors, interior temperature sensors, outlet duct and evaporator temperature sensors, pressure sensors, humidity sensors, blend door position sensors and sunload sensors. On the 2004 Acura TL there's even a global positioning sensor (GPS) that allows the control module to determine the vehicle's orientation to the sun so it can increase cooling on the side facing the sun and decrease cooling on the shaded side. Some vehicles also use passive infrared sensors mounted in the dash or an overhead console t...