Maximum Heater Output Test

Maximum Heater Output Test

Engine coolant is delivered to the heater core through two heater hoses. With the engine idling at normal operating temperature, set the temperature control to the full hot position, the mode control to the floor position, and the blower motor control to the highest speed position. Using a test thermometer, check the temperature of the air being discharged at the front HVAC housing floor outlets or the rear outlet in the passenger side rear quarter interior trim panel. Compare the test thermometer reading to the Heater Temperature Reference chart.

Heater Temperature Reference






Both of the heater hoses should be hot to the touch. The coolant return heater hose should be slightly cooler than the coolant supply heater hose. If the return hose is much cooler than the supply hose, locate and repair the coolant flow obstruction or mechanical problem.

Obstructed Coolant Flow

Possible locations or causes of obstructed coolant flow are as follows:

• Inoperative water pump.

• Inoperative engine thermostat (stuck or sticking closed).

• Pinched or kinked heater hoses.

• Improper heater hose routing.

• Plugged heater hoses or supply and return ports at the cooling system connections.

• Plugged heater core or heater core tubes.

If proper coolant flow through the cooling system is verified, and heater outlet air temperature is still low, a mechanical problem may exist.

Mechanical Problems

Possible locations or causes of insufficient heat due to mechanical problems are as follows:

• Low coolant level (coolant leak or loose or missing pressure cap).

• Obstructed cowl air intake.

• Obstructed heater system outlets.

• Inoperative engine thermostat (stuck or sticking open).

• Blend-air doors or actuators not functioning properly.

• Inoperative blower motor system.

• Inoperative A/C heater control.

Temperature Control

If the heater outlet air temperature cannot be adjusted with the temperature controls on the A/C heater control, the following could require service:

• Inoperative A/C heater control.

• Inoperative A/C heater control.

• Inoperative blend door actuators.

• Inoperative, obstructed or improperly installed blend-air doors.

• Inoperative related wiring harness or faulty connections.

• Improper engine coolant temperature.