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 intermitting problems. When an intermitting condition is suspected, inspect the suspected circuits for the following conditions:

History DTCs are those indicating that the HVAC Control Head 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 Head. 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 Head operation with no DTC stored:

The following conditions must be met before you can perform the functional checks below:

1. The engine must be running.
2. The engine temperature must be at least 63°C (145°F).
3. You must start from the AUTO mode.

The passenger temperature control is over-ridden and AUTO will not be displayed if:

The outside air temperature display is updated when:

When the outside air temperature is less than 2°C (35°F), the A/C compressor will not engage.

When the outside air temperature is between 2°C and 7°C (35°F and 45°F), the A/C compressor might not engage.