Heating Ventilation Air Conditioning (HVAC)

Fig. 1 ECM diagnostic codes:

Fig. 2 BCM diagnostic codes:

The ECM and BCM continually monitor operating conditions of their controlled systems for possible malfunctions. When a problem is detected, a two-digit numerical "trouble code," Figs. 1 and 2, is stored within the computer memory. These codes can be displayed as aid in system repair. Any malfunctions which the vehicle operator should be aware of will illuminate Service Now or Service Soon warning lamps on the dashboard.

If a malfunction which would result in unacceptable system operation is encountered, the self-diagnostics will minimize the effect by taking "fail-soft" action to compensate for the detected problem. A typical "fail-soft" action, for example, would be the substitution of a fixed input value when a sensor is detected to be open or shorted.