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.

SETTING FAULTS - ATC AND MTC SYSTEMS
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 for both heating-A/C systems continuously monitors various internal parameters during normal system operation. If the control 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 setting and resetting of some active DTCs requires a wait time. Both the MTC and the ATC A/C-heater controls use two independent wait times for setting faults and one for resetting faults. The three different wait times are as follows:
- Start Up Wait Time - This is only used immediately after an ignition transition from low to high. It defines how long the system waits after the ignition line has gone high before setting any active DTCs. The duration of the Start Up Wait Time is 1.0 second.
- Set Wait Time - This is used at the conclusion of the Start Up Wait Time. It defines the duration a monitored parameter must remain continuously outside of the acceptable range before an active DTC will set. The duration of the Set Wait Time is 1.5 seconds.
- Reset Wait Time - It defines the duration a monitored parameter must remain continuously within the acceptable range before an active DTC resets. The duration of the Reset Wait Time is 1.5 seconds.

STORING FAULTS - ATC AND MTC SYSTEMS
The A/C-heater control for both the ATC and MTC systems stores the most recent fault by code number in data record number 1. For each new fault, the data records push up one level. After recording eight faults, the system deletes the oldest from the stack. If a fault clears in the middle of the stack due to key cycles, all of the faults below move up one level. To prevent a single fault from filling up the record, the system will only capture an active fault once upon its first detection. However, the system will record an active fault again if is clears and then reoccurs, such as in the case of an intermittent failure condition.