On non-ATC systems, the LED light on the rear-window defogger button will display some diagnostic codes by blinking.
1) To begin the diagnostic mode, run the vehicle at idle with the controls set as followed:
Blower Speed Knob - Any position other than OFF
Temperature Knob - FULL COOL
Right Knob - Defrost Mode
2) Then, press the defogger switch in for 5 seconds until it begins to blink, then release it. It will continue to blink for about 30 seconds, if the light goes out, calibration is OK, if calibration is Okay skip to step 6.
3)If the light remains blinking, switch the mode knob to the MIX position, if the light still remains on there is a problem with the control head.
4) Then switch to FLOOR position, if the light stays on it indicates a problem with the blend door or the blend actuator wiring.
5) Then switch to BI-LEVEL position, if the light stays on it indicated a problem with the mode actuator, or circuit.
6) If the light went off, move the knob to the MIX mode, the LED should flash twice then pause, and continue the 2 flash sequence indicating that this mode is OK.
7) In the FLOOR mode, the light should flash in threes, indicating this mode is OK. In BI-LEVEL mode, a 4-flash sequence indicates this mode is OK. In RECIRCULATE-PANEL mode, a 5-flash sequence means OK, and in RECIRCULATE-BI-LEVEL mode a 6-flash sequence means OK.
8) The temperate KNOB can be checked by switching the mode knob to DEFROST and rotating the temperature knob, the light will flash faster as you turn the temperature knob up, and will turn off at full HOT setting, indicating this mode is OKAY.
9) Switch the KNOB between Panel and Re-circulate Panel and watch the recirculation door, proper operation means the circuit and components are OK..
To get OUT of SELF Diagnostic mode, push the A/C button once.