Symptom Related Diagnostic Procedures

There are several thermal and mechanical problems that can prevent the Automatic Temperature Control (ATC) system from properly controlling the interior comfort of the vehicle. Refer to the following list of the most common symptoms and possible causes.

The system works fine at night but does not seem to properly control temperatures in partly sunny weather (sun intensity changes).
The sun sensor circuit that has failed open or an improperly installed instrument panel top cover will cause this problem. The body control module cannot detect a sun sensor or wiring that has an open circuit and no trouble code will be recorded. A sun sensor should have a resistance of 1 to 5 mega ohms. The instrument top cover can block the sun sensor if it covers too much of the sensor. The top cover when properly installed should allow the sun sensor to stick out .25 inch (measured from the top of the sensor to the cover).

NOTE: Items left on the instrument panel that cover the sun sensor can also affect the operation of the system.

The system seems to cycle the interior temperature too hot or cold (erratic temperature control).
This problem can be caused when the in-car temperature sensor does not sense the temperature of the passenger compartment. This occurs when the in-car temperature sensor aspirator motor does not operate or it certain instrument panel seals are not properly installed. This problem is more noticeable when the weather is very cold or hot. Small air leaks of cold or hot air from areas other than the in-car temperature sensor inlet (located behind the name plate) will make the sensor measure the wrong temperature. Refer to Diagnostic Charts/Test 9A to diagnose the in-car temperature sensor. 9A

The ATC control panel blue display goes blank and the ATC control operates like a manual A/C-heater control.
This will occur if there is a problem with the in-car temperature sensor thermistor or vehicle wiring. Note that an in-car temperature sensor trouble code will be present. Refer to Diagnostic Charts/Test 9A for repair. 9A

The system does not seem to cool the vehicle.
This can occur if the ATC-heater unit temperature door is not properly calibrated. Start the engine and perform the ATC-heater unit door calibration with the DRB. Also check the performance of the ATC-heater unit by performing a partial charge check with the DRB.

The panel outlet temperatures seem to cycle.
Use the DRB to position the temperature door at a fixed position. If the panel outlet air does not cycle. If the outlet temperature continues to cycle excessively, check the operation of the evaporator sensor and the A/C system expansion valve (TXV).

The air that leaves the ATC-heater unit does not correspond to what the LEDs indicate on the ATC display.
Check to see of the ATC control has a communications trouble code. If no trouble code is present, check the operation of the ATC-heater mode door actuator linkages.