Important Preliminary Inspections Before Starting

Before using the Symptom diagnosis, perform the following:

•	Perform Diagnostic System Check - Vehicle and verify all of the following items:

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Ensure that the engine control module (ECM) and indicator lamp are operating correctly.

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Ensure that there are no diagnostic trouble codes (DTCs) that are stored.

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The scan tool data is within the normal operating range. Refer to Control Module References for scan tool information.

•	Verify the customer concern.

•	Perform the Visual/Physical Inspection in this section. The visual/physical inspection is extremely important, and can lead to correcting a condition without additional testing. It may also help reveal the cause of an intermittent condition.

•	Locate the correct symptom. Perform the tests and inspections associated with the symptom.

Review the system operation in order to familiarize yourself with the system functions. Refer to Cooling Fan Description and Operation (LHU/A20NFT, LAU/A28NER).

Visual/Physical Inspection

Several of the symptom procedures call for a careful visual and physical inspection. This can lead to correcting a condition without further tests and can save time. This inspection should include the following areas:

•	Ensure that the control module grounds are clean, tight, and correctly located.

•	Inspect cooling system hoses and pipes for splits, kinks, and proper connections. Inspect thoroughly for any type of a leak or a restriction.

•	Inspect for a dirty or restricted radiator and HVAC condenser.

•	Inspect for aftermarket devices which could affect the operation of the Cooling System.

•	Inspect the easily accessible or visible system components for obvious damage or conditions which could cause the symptom.

•	Inspect the surge tank reservoir for the proper coolant level.

Identifying Intermittent Conditions

Many intermittent conditions occur with harness or connector movement due to engine torque, rough pavement, vibration or physical movement of a component. Refer to the following for a list to help determine an intermittent condition:

•	Moisture and water intrusion in connectors, terminals, and components

•	Connector mating

•	Terminal contact

•	High resistance in circuits or components. High resistance can be a resistance, irrespective of size, that disrupts the function of components.

•	Harness that is located too tight, or chaffed circuits

•	High or low ambient temperature

•	High or low engine coolant temperatures

•	High underhood temperatures

•	Heat build up in component or circuit due to circuit resistance, poor terminal contact, or high electrical load

•	High or low system voltage

•	High vehicle load conditions

•	Rough road surface

•	Electro-magnetic interference (EMI)/circuit interference from relays, solenoids or other electrical surge

•	Incorrect installation of non-factory, aftermarket, and after factory add on accessories

If an intermittent is determined, refer to Testing for Intermittent Conditions and Poor Connections for specific strategies in diagnosing intermittent conditions.

Symptom List

Refer to a symptom diagnostic procedure from the following list in order to diagnose the symptom:

•	Engine Overheating

•	Loss of Coolant

•	Thermostat Diagnosis

•	Engine Fails To Reach Normal Operating Temperature