To Enter the Diagnostic Mode:

1. Press the "EXT TEMP" button then press the "ECON" button. Hold both buttons for three seconds. -00 will be displayed, indicating entry to the diagnostic mode.
2. -00 is also called a diagnostic pointer. A diagnostic pointer indicates the parameter which can be displayed. To move through the list, press "FAN" button. To display the information in a diagnostic pointer, press the "EXT TEMP" button.
3. Diagnostic pointers -40-41 and -42 are output commands which allow you to control the position of the Air Mix Valve, the program number or the air delivery mode. To enter an output command, press the "EXT TEMP" button when the desired pointer number is displayed, then use the "TEMP" button to change the commanded value.
4. To exit the diagnostic mode, press the "OFF" button. To exit from a diagnostic pointer, but not from the diagnostic mode, press the "FAN" button. Note: A two-digit trouble code indicates the fault is present now. A three-digit trouble code indicates the fault was present. Clearing Codes

o Clear codes from the Heater And A/C Control Assembly by removing Fuse 11.

To summarize, press & hold ECON and EXT TEMP for 3 seconds until -00 is displayed. Then press EXT TEMP to view codes. Press OFF to leave diagnostic mode.
00 = No system faults
01 or 101 = In-car temp sensor short
02 or 102 = In-car temp sensor open
03 or 103 = Ambient temp sensor short
04 or 104 = Ambient temp sensor open
05 or 105 = Air mix valve feedback circuit open