For 1990-2009 cars

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
Important: Insufficient heating may be caused by the following conditions:

• | A low coolant level

• | A faulty thermostat

• | A partially or fully clogged heater core or heater hose

• | An obstruction to air curculation

• | An air leak past a faulty seal into the passenger compartment

1 | Inspect for the following conditions:

• | Low coolant level

• | Loose or worn belts

• | Leaking or kinked hoses

• | Missing radiator cap pressure seal

Are any of these conditions present?

Repair as necessary.

Is the repair complete?

--

System OK

Go to Step 2

2 |

1. Adjust the heater controls to the following settings:

1.1. | Heater mode

1.2. | High blower speed

1.3. | Full warm

2. Idle the vehicle.
3. Inspect the floor air outlet.
4. Inspect the outlet attachment.

Is there high air flow?

--

Go to Step 3

Go to Step 16

3 | Inspect the blower speed for airflow change. Does the speed of the airflow change? | -- | System OK | Go to Step 4
4 |

1. Inspect the blower switch and the resistor.
2. Repair the blower switch and the resistor.
3. Complete the heater output temperature check.

Is the heater output temperature correct?

--

Go to Step 5

Go to Step 6

5 |

1. Inspect the vehicle for cold air leaks at the following locations:

• | The cowl

• | The heater case

• | The vents

2. Inspect under the seat for obstructions.

--

--

--

6 |

1. Place the ignition switch in the RUN position.
2. Place the blower on LOW.
3. Rotate the temperature control knob to the mid position.
4. Rotate the knob rapidly to the full HOT position.
5. Listen for valve slam at the end of the control knob rotation.

Is there a valve slam?

--

Go to Step 9

Go to Step 7

7 |

1. Inspect the temperature control cable for correct attachment at the control and the heater module.
2. Repair as necessary.

Is there valve slam?

--

Go to Step 9

Go to Step 8

8 | Replace the temperature control cable. Refer to Temperature Control Cable Replacement . Is the repair complete? | -- | System OK | --
9 |

1. Place the temperature valve in full WARM.
2. Start the vehicle.
3. Ensure that the air temperature around the hoses is at least 24°C (85°F).
4. Inspect the temperature of the heater inlet and outlet hoses by feel.

Is the inlet hose hotter than the outlet hose?

--

Go to Step 13

Go to Step 10

10 |

1. Back flush the heater core.
2. Drain and refill the coolant. Refer to Cooling System Draining and Filling in Engine Cooling.
3. Retest.

Is the inlet hose hotter than the outlet hose?

--

Go to Step 11

Go to Step 12

11 | Repeat the test. Do the symptoms still exist? | -- | -- | --
12 | Replace the heater core. Is the repair complete? | -- | System OK | --
13 | Inspect the thermostat for proper installation and seating. Are the installation and seating correct? | -- | Go to Step 14 | Go to Step 15
14 | Replace the thermostat. Refer to Engine Coolant Thermostat Replacement in Engine Cooling. Is the repair complete? | -- | System OK | --
15 |

1. Reinstall the thermostat.
2. Inspect the thermostat.

--

--

--

16 | Run the Functional Check . Is the condition corrected? | -- | System OK | Go to Step 17
17 | Inspect the airflow from the defroster and vent outlets. Is there high airflow from the defroster or vent outlets? | -- | Go to Step 18 | Go to Step 19
18 |

1. Inspect the vacuum system.
2. Repair the system as necessary.

Is the repair complete?

--

System OK

--

19 |

1. Switch the mode control knob to defrost.
2. Inspect the airflow.

Is the defroster airflow correct?

--

Go to Step 20

Go to Step 21

20 | Inspect the floor air outlet for obstructions. | -- | -- | --
21 | Inspect the blower speeds for airflow change. Does the speed of the airflow change? | -- | Go to Step 22 | Go to Step 26
22 | Inspect for airflow obstructions at blower inlet and plenum. Are there obstructions at the blower inlet and the plenum? | -- | -- | Go to Step 23
23 |

1. Place the blower on HIGH.
2. Move the temperature control knob from full WARM to full COOL.
3. Listen to the airflow.

Does the speed of the airflow change?

--

Go to Step 24

Go to Step 25

24 | Inspect for airflow obstruction between the blower and the system outlets. Is there an obstruction? | -- | Go to Step 26 | Go to Step 25
25 |

1. Inspect the following components:

• | Temperature valve

• | Temperature cables

• | Temperature control

2. Repair the components as necessary.

Are the repairs complete?

--

System OK

--

26 |

1. Inspect the blower switch and resistor.
2. Repair as necessary.

Are the repairs complete?

--

System OK

--