Reading and Clearing Diagnostic Trouble Codes

DIAGNOSTIC TROUBLE CODES (DTC's)
Both the Automatic Temperature Control (ATC) and the Manual Temperature Control (MTC) systems are controlled by the Body Control Module (BCM). Both systems can be diagnosed by the DRB III scan tool or the vehicles own control head display. Refer to the DRB III menu for checking Diagnostic Trouble Codes (DTC's). Note that there are three DTC tables. The ATC and MTC DTC table contain faults that are common to both the ATC and the MTC system. The same diagnosis can be used for both systems. The DTC's cover operation of the climate control unit actuators, doors, evaporator temperature sensor, ambient temperature sensor and the A/C refrigerant system. The MTC DTC table covers Fault Codes that are for the MTC Control Head and wiring and are not used on an ATC system. The ATC DTC table has DTC's for ATC Head Communications, In-Car Temperature Sensor and Sun Sensor which are not in a MTC system.

Some conditions of low battery voltage or extremely cold weather can generate a DTC for the climate control system actuators, evaporator probe, in-car temperature sensor (ATC), or ambient temperature sensor, even in a properly operating system. It is recommended that all the DTC's be checked as follows to assure a part is faulty.

Climate Control Sensor DTC Check
1. Clear the DTC.
2. Operate the HVAC system with the engine running for a minimum of 30 seconds.
3. Check for DTC's.
4. Perform diagnostics if the DTC appears again.

Climate Control Actuator Check.
1. Clear the DTC.
2. Perform HVAC system calibration.
3. Perform diagnostics if the DTC appears again.

If a DTC does not appear when using the procedures above, check the history of the vehicle for a low battery condition, or ambient temperatures below 40 °F. Removing connections without disconnecting the battery may also generate a DTC on a good part. The battery should always be disconnected when servicing electrical parts.

Trouble Codes From The ATC Head

ATC And MTC DTC's:

ATC DTC's:

The trouble codes can be checked with the ATC control if a DRB III scan tool is not available. The control head can only be placed into the diagnostic mode while the engine is running and the vehicle is not moving. Set the control to a 75 °F setting (so there is no confusion with the 23 - 51 Diagnostic Trouble Codes (DTC's).

Fig.1 Entering Self-Diagnostic Mode:

To place the system into it's diagnostic mode, press and hold the Floor, Mix and Defrost buttons (at the same time). The ATC head display will begin to blink. Release the Floor, Mix and Defrost buttons. Once the control head enters the diagnostic mode, the display on the control head will continue to blink. This occurs until it completes its tests and climate control unit door/actuator calibrations. Then it will display any diagnostic trouble codes that are present in the BCM. If there are no diagnostic trouble codes, the system will return to its normal operation as indicated by the temperature symbol (C or F). Diagnostic trouble codes related to the ATC and climate control unit will appear on the display in numerical form. The diagnostic trouble codes are stored in the BCM and can range between 23 and 51. The ATC control can only show one diagnostic trouble code at a time. Under certain circumstances, more than one diagnostic trouble code could be in the memory. To scroll through any additional diagnostic trouble codes, press the Panel Mode button on the ATC control. It is important that no other knob or button is pushed until all codes are read. Pushing any button except Panel Mode or turning a knob will end the diagnostic test without showing the rest of the trouble codes. There also may be other trouble codes stored in the BCM not related to the Climate Control System. These codes can only be found using the DRB III scan tool.

Trouble Codes From The MTC Head

Fig.2 MTC Control Head:

The trouble codes can be checked with the MTC head and the Odometer Display of the Mechanical Instrument Cluster if a DRB III scan tool is not available. The MTC Head can only be placed into the diagnostic mode while the engine is running and the vehicle is not moving. To place the system into it's diagnostic mode, adjust the MTC to the following settings:
1. The engine must be running with vehicle not moving.
2. The Fan Speed set to any speed except OFF.
3. The Temperature Knob in full cold (full counterclockwise position).
4. The mode knob must be placed in defrost position (full clockwise position).
5. The A/C button can be ON or OFF.
6. Press and hold the EBL button until the Mechanical Instrument Cluster Odometer display indicates an "AC00". The body control module (BCM) will chime once and the MTC A/C button LED will begin blinking.
7. Release the EBL button and wait until the MTC A/C button LED stops blinking. This means that the error check and climate control unit door calibration is complete.

MTC DTC's:

Diagnostic trouble codes related to the MTC head and the climate control unit will appear on the odometer display in numerical form after the letters "AC". The diagnostic trouble codes are stored in the BCM and can range between 23 and 51. The odometer display will return to normal operation if no trouble codes are found. If a problem is found, then the odometer will display the letters "A/C" followed by the trouble code number. The odometer display can only show one diagnostic trouble code at a time. Under certain circumstances, more than one diagnostic trouble code could be in the memory. To scroll through any additional diagnostic trouble codes, press the A/C button on the MTC head. The BCM will beep each time the A/C button is pushed. Continue pushing the A/C button and recording the trouble code numbers until the odometer returns to normal operation (note the letters AC will disappear).

It is important that no other knob or button is pushed until all codes are read. Pushing any button except the A/C button or turning a knob will end the diagnostic test without showing the rest of the trouble codes. There also may be other trouble codes stored in the BCM not related to the Climate Control System. These codes can only be found using the DRB III scan tool.

Erasing Diagnostic Trouble Codes (DTC's)
Diagnostic trouble codes can be cleared from the memory two ways:
- The DRB III scan tool
- Power to the BCM can be disconnected for ten minutes by disconnecting the battery negative remote cable.

If the scan tool is to be used, refer to the Appropriate Diagnostic Information.

CAUTION:
- Fault code 26 can be created if the in-car sensor thermistor is disconnected while the BCM is operating. This can happen by disconnecting connectors from the BCM or the ATC Control Head when the battery is connected (even if the ignition switch is OFF). To correct, clear the fault code 26 from the BCM.