GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2002 🢒 Sonoma Pickup - 4WD 🢒 Engine 🢒 Engine Cooling 🢒 Heating Performance Diagnostic

Step

Action

Yes

No

1

Were you sent here from Too Cold in Vehicle?

Go to Step 2

Go to in HVAC Systems - Manual

2
  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 9

3

    Caution: Avoid contact with moving parts and hot surfaces while working around a running engine in order to prevent physical injury.

  1. Allow 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 at the heater core.

Does the inlet hose feel warmer than the outlet hose?

Go to Step 7

Go to Step 4

4
  1. Install a thermometer into the I/P 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:
    7. • The I/P center air outlet
    • The heater core outlet hose
  7. Compare the recorded temperatures.

Are the temperatures similar?

Go to Step 5

Go to Step 6

5
  1. Inspect and repair the following areas of the vehicle for cold air leaks:
    2. • The cowl
    • The recirculation door
    • The HVAC module case
  2. Perform the necessary repairs.

Are the repairs complete?

Go to Step 10

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6
  1. Inspect the temperature door operation. Refer to Diagnostic System Check - HVAC Systems - Manual
  2. Perform any necessary repairs.

Are the repairs complete?

Go to Step 10

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7
  1. Turn OFF the engine.
  2. Backflush the heater core. Refer to Flushing
  3. Start the engine.
  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 8

Go to Step 10

8

Replace the heater core. Refer to Heater Core Replacement .

Is the repair complete?

Go to Step 10

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9

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 10

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10

Operate the system in order to verify the repair.

Does the system operate correctly?

System OK

Go to Step 2