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

Does the engine reach a normal operating temperature?

Go to Step 2

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 temperature setting.
3. Select the minimum blower speed.
4. Feel the temperature of the inlet and outlet hoses to the hot water bypass valve.

Does the inlet hose feel warmer than the outlet hose?

Go to Step 3

Go to Step 6

Disconnect the vacuum hose from the hot water bypass valve.

Is vacuum being applied to the valve?

Go to Step 13

Go to Step 4

Remove the hot water bypass valve. Refer to Heater Water Valve Replacement .

Inspect the hot water bypass valve for the following conditions:

Are any of the listed conditions present?

Go to Step 12

Go to Step 5

Install the hot water bypass valve. Refer to Heater Water Valve Replacement .

Is the action complete?

Go to Step 6

--

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

Does the inlet hose feel warmer than the outlet hose?

Go to Step 10

Go to Step 7

1. Install a thermometer into the I/P panel center air outlet.
2. Secure a thermometer to the heater core outlet hose.
3. Select the panel mode.
4. Select the maximum blower speed.
5. Select the maximum temperature setting.
6. Record the temperature at the following locations:

•	The I/P panel center air outlet

•	The heater core outlet hose

7. Compare the recorded temperatures.

Are the temperatures similar?

Go to Step 8

Go to Step 9

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

•

The cowl

•	The recirculation door

•	The HVAC module case

2. Perform the necessary repairs.

Are the repairs complete?

Go to Step 15

--

1. Inspect the temperature door operation. Refer to Diagnostic System Check .
2. Perform any necessary repairs.

Is the repair complete?

Go to Step 15

--

1. Turn OFF the engine.
2. Backflush the heater core. Refer to Flushing in Engine Cooling.
3. Start the engine and allow to idle.
4. Select the FLOOR mode.
5. Select the minimum blower speed.
6. Select the maximum temperature setting.
7. Feel the temperature of the inlet and outlet hoses at the heater core.

Does the inlet hose feel warmer than the outlet hose?

Go to Step 11

Go to Step 15

Replace the heater core. Refer to Heater Core Replacement .

Is the repair complete?

Go to Step 15

--

Replace the hot water bypass valve. Refer to Heater Water Valve Replacement .

Is the repair complete?

Go to Step 15

--

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

Is the repair complete?

Go to Step 15

--

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 15

Operate the system in order to verify the repair.

Did you correct the condition?

System OK

Go to Step 1