Intermittent failures in electronic systems may be very
difficult to detect and accurately diagnose. The heater A/C control tests
for different malfunctions under different vehicle conditions. For this reason,
a through test drive is often needed in order to repeat a fault condition.
If the system malfunction is not repeated during the drive test, a good description
of the complaint may be useful in locating an intermittent fault condition.
Faulty wires or connections cause most intermittent problems. When an intermittent
condition is suspected, inspect the suspected circuits for the following
conditions:
| • | Poor mating of connector halves of backed out terminals. |
| • | Improperly formed or damaged terminals. |
| • | Poor wire-to-terminal connections. |
| • | Dirty or corroded terminals. |
History Diagnostic Trouble Codes (DTCs) are those indicating that the
heater A/C control has previously detected a malfunction. The reason could
be that the malfunction is intermittent and cannot be found easily. Try clearing
the DTCs and operate the component that the DTC applies to by using a scan
tool. Read the diagnostic aids and conditions for setting the DTC in each
DTC table for more specific information.
History DTCs help in diagnosing intermittent fault conditions, but do
not necessarily indicate a faulty heater-A/C control. Most system complaints
are linked to faulty wiring and connectors, and occasionally to a component.
Perform a thorough inspection of all related wiring and connectors pertaining
to the history DTC that are stored.
The following conditions may result in the intermittent heater A/C control
operation with no DTCs stored:
| • | Any condition which results in the interruption of power to the
heater A/C control. |
| • | The battery or ignition voltage is out of range (below 9V or above
16V). |
| • | A loose or damaged ground. |
| • | An open or short in the Class 2 serial data link. |
