DEFINITION: Heating system performance of the auxiliary heater.

Were you sent here from Symptoms or another diagnostic table?

Go to Step 2

Go to Symptoms - HVAC Systems - Manual or Symptoms - HVAC Systems - Automatic

1. Start the engine.
2. Allow the engine to idle.

Does the engine reach a normal operating temperature?

Go to Step 3

Go to Step 14

Caution: Refer to Moving Parts and Hot Surfaces Caution in the Preface section.

1. Allow the engine to idle.
2. Select the maximum auxiliary temperature setting.
3. Select the minimum auxiliary blower speed.
4. Feel the temperature of the inlet and outlet heater hoses to the auxiliary hot water bypass valve.

Does the inlet heater hose feel warmer than the outlet heater hose?

Go to Step 4

Go to Step 6

Disconnect the vacuum hose from the coolant bypass valve.

Is vacuum being applied to the valve?

Go to Step 13

Go to Step 5

1. Remove the coolant bypass valve. Refer to Coolant Bypass Valve Replacement .
2. Inspect the coolant bypass valve for the following conditions:

•

Binding

•	Restrictions

•

Damage

Are any of these conditions present?

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Go to Step 16

1. Allow the engine to idle.
2. Select the minimum auxiliary blower speed.
3. Select the warmest auxiliary temperature setting.
4. Feel the temperature of the inlet and outlet heater hoses to the auxiliary heater core.

Does the inlet heater hose feel warmer than the outlet heater hose?

Go to Step 10

Go to Step 7

1. Install a thermometer into an auxiliary air outlet.
2. Secure a thermometer on the outlet heater hose of the auxiliary heater core.
3. Select the maximum auxiliary blower speed.
4. Select the warmest auxiliary temperature setting.
5. Record the temperature at the following locations:

•	The auxiliary air outlet

•	The auxiliary heater core outlet hose

6. Compare the recorded temperatures.

Are the two temperature readings about equal?

Go to Step 8

Go to Step 9

1. Inspect the following areas of the vehicle for cold air leaks:

•	The auxiliary HVAC module case

•	The auxiliary air ducts

2. Perform any necessary repairs.

Is the repair complete?

Go to Step 16

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Inspect the auxiliary HVAC module for correct temperature door operation. Refer to Diagnostic System Check - HVAC Systems - Manual or Diagnostic System Check - HVAC Systems - Automatic .

Does the temperature door operate correctly?

Go to Step 16

Go to Step 15

1. Turn OFF the engine.
2. Backflush the auxiliary heater core.
3. Start the engine and allow the engine to idle.
4. Select the minimum auxiliary blower speed.
5. Select the warmest auxiliary temperature setting.
6. Feel the temperature of the inlet and outlet heater hoses at the auxiliary heater core.

Does the inlet heater hose feel warmer than the outlet heater hose?

Go to Step 11

Go to Step 16

Replace the auxiliary heater core. Refer to Heater Core Replacement .

Is the repair complete?

Go to Step 16

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Replace the coolant bypass valve. Refer to Coolant Bypass Valve Replacement .

Is the repair complete?

Go to Step 16

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Repair the vacuum condition. Refer to Vacuum Control System Diagnostic in HVAC Systems - Manual or Vacuum Control System Diagnostic in HVAC Systems - Automatic.

Is the repair complete?

Go to Step 16

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Repair the low engine temperature concern. Refer to Engine Fails To Reach Normal Operating Temperature in Engine Cooling.

Is the repair complete?

Go to Step 16

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Repair the temperature door operation.

Is the repair complete?

Go to Step 16

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Operate the system in order to verify the repair.

Did you find and correct the condition?

System OK

Go to Step 2