For 1990-2009 cars

Intermittent failures in electronic systems may be very difficult to detect and accurately diagnose. A good description of the complaint may be useful in locating an intermittent malfunction. Faulty wires or connections cause most intermitting problems. When an intermitting 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

• | Wire chafing

• | Poor wire to terminal connections

• | Dirty or corroded terminals

• | Connector body damage

History DTCs are those indicating that the heater and A/C control system has previously detected a malfunction. The reason could be either that the malfunction is intermittent or that the conditions for setting the DTC are not met. Try clearing the DTCs and manually operate the system that the DTC applies to. 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 and A/C control unit. The control unit 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 heater and A/C control operation with no DTC stored:

• | Any condition which results in the interruption of power to the heater and A/C control.

• | 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.