Intermittent failures in electronic systems may be very
difficult to detect and accurately diagnose. For this reason, a thorough test
drive is often needed in order to repeat a fault condition. If the system
malfunction is not repeated during the test drive, a good description of
the complaint may be useful in locating an intermittent malfunction. 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 DTCs are those indicating that the HVAC Control Assembly has
previously detected a malfunction. The reason could be either that the malfunction
is intermittent or that the system for which the DTC is set for is not
currently being operated. History DTCs help in diagnosing intermittent fault
conditions but do not necessarily indicate a faulty HVAC Control Assembly.
The HVAC Control Head is a reliable component, and is not the likely cause
of the malfunction. 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
is stored.
The following conditions may result in intermittent HVAC Control Assembly
operation with no DTC stored:
| • | Any condition which results in the interruption of power to the
HVAC Control Assembly. |
| • | The battery or ignition voltage is out of range (below 9.0 volts
or above 16.0 volts). |
| • | A loose or damaged ground. |
| • | An open or a short on the serial data line. |
