For 1990-2009 cars

The General Motors fully automatic temperature controller (FATC) contains a self-diagnosis function to aid in finding any problem with the system. To enter the diagnostic mode, perform the following procedure:

1. Turn the ignition to ON.
2. Set the temperature control to 26°C (79°F).
3. Within 3 seconds, push the AUTO and the OFF switches simultaneously, more than 3 times.
4. Count the number of times the temperature indicator screen blinks.
5. If there are no error codes set, the screen will not blink. When the controller indicates an error code, perform the diagnostic for the displayed DTC. Refer to Diagnostic Trouble Code (DTC) List.
6. Push the OFF switch to return the controller to its normal functions.