Diagnostic Strategies

Begin the system diagnosis by reviewing the system Description and Operation. Reviewing the Description and Operation information will help you determine the correct symptom diagnostic procedure when a malfunction exists. Reviewing the Description and Operation information will also help you determine if the condition described by the customer is normal operation. Refer to Symptoms in order to identify the correct procedure for diagnosing the system and where the procedure is located.





Before beginning the functional test of the heater system, the engine must idle with the coolant hot and the thermostat open. The coolant temperature must be about 90 °C (194 °F). During the function test, evaluate the operating efforts of the mode control knob and the temperature control knob. Even if a problem is found during any intermediate step of the function test, complete the test before the repair is started.