For 1990-2009 cars

### Circuit Description

The calibrations for the heater and A/C control are stored in EEPROM. When the ignition is first turned On, the controller performs internal tests on the EEPROM and can determine the integrity of the EEPROM's non-volitale memory. The controller compares its previously stored checksum with the checksum calculated when the ignition was first turned On. If the two checksums do not match, the controller indicates that a calibration error (checksum error) has occured.

### Conditions for Setting the DTC

• | The stored checksum does not match the calculated checksum.

• | The controller detects an internal memory malfunction (checksum error).

• | This condition must occur when the ignition is first turned On.

### Action Taken When the DTC Sets

• | Stores a DTC 51 in the controllers memory.

• | Disables all heater and A/C control functions.

### Conditions for Clearing the DTC

• | The condition for the fault is no longer present.

• | The use of a scan tool.

• | A history DTC will clear after 20 consecutive ignition cycles if the condition for the fault is no longer present.

### Diagnostic Aids

Because the EEPROM is not removable from the heater and A/C control and a DTC 51 is an internal malfunction to the controller, the repair procedure is to replace the heater and A/C control. Program the new heater and A/C control. Refer to Heater and A/C Control Replacement for the programming procedure.