For 1990-2009 cars

### Circuit Description

DTC 55 is an internal control unit hardware failure (RAM, ROM defective) in the heater and A/C control.

### Conditions for Setting the DTC

• | The heater and A/C control detects an internal hardware malfunction.

• | The heater and A/C control checks the hardware on a continuous basis every 200 ms.

### Action Taken When the DTC Sets

• | Stores a DTC 55 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.

### Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

1. The hardware for the controller is internal and cannot be replaced. The repair procedure is to replace the heater and A/C control module.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
Schematic Reference: HVAC Schematics
1 | Did you perform the HVAC Systems Diagnostic System Check? | -- | Go to Step 2 | Go to Diagnostic System Check - HVAC Systems - Automatic
2 | Important: Perform the reprogramming procedure for the HVAC control module Replace the HVAC control module. Refer to HVAC Control Module Replacement . Did you complete the replacement? | -- | System OK | --