Initial Inspection and Diagnostic Overview

CAUTION: Do not exchange A/C - Heater Controls from vehicle to vehicle. Software versions differ between models and model years. Installing an A/C - Heater Control with software that is incompatible for a given vehicle can result in either improper or failed HVAC system operation.

Both the automatic temperature control (ATC) and the manual temperature control (MTC) systems are controlled by the A/C - heater control located on the center bezel of the instrument panel. The ATC system communicates on the controller area network (CAN) B bus and is fully addressable with a scan tool. The MTC system is NOT connected to the CAN bus and is NOT addressable with a scan tool. The A/C - heater control's primary means of fault detection for both heating - A/C systems is through active and stored diagnostic trouble codes (DTCs). The control continuously monitors various internal parameters during normal system operation. If it detects a fault, both an active and a stored diagnostic trouble code (DTC) will set for that parameter. When the offending parameter returns to an acceptable value, the control automatically clears the active DTC. However, the stored DTC remains until cleared (either manually or automatically). Note that DTCs will not set or clear if supply voltage is low.

The A/C - heater control's secondary means of fault detection for both heating - A/C systems is through system tests. For ATC systems, these tests include the HVAC System Test, the Cooldown Test, and Actuator Calibration. For MTC systems, these tests include the HVAC System Test, the Cooldown Test, Actuator Calibration (new MTC installation only), and the Actuator Circuit Test / Door Calibration Function. Refer to System Tests for a detailed description of each test.