No heat in car — Most common causes and fixes

Learn what to check if your car has no heat

Driving in cold weather without a functioning heater can be quite uncomfortable and even hazardous. If you find yourself shivering in your car, it’s important to diagnose and fix the issue as soon as possible. Here are some key areas to check if your car has no heat.

1) A low coolant level is the most common cause of no heat in car

One of the most common reasons for a car’s heater to stop working is a low coolant level. Coolant, circulates through your hot engine, picks up heat, and then flows to your heater core to provide cabin heat. If the coolant level is too low, the heater core won’t receive enough coolant flow to generate heat. To check the coolant level, open the hood and locate the coolant reservoir. Ensure the fluid is at the proper level indicated on the tank. Remember to never open the radiator cap when the engine is hot, as this can cause serious burns.

2) A faulty blend door or blend door actuator can cause no heat in car or cool air

The blend door is what regulates the air temperature coming out of the vents. As the name implies, it blends hot air coming from the heater core with outside air and recirculated cabin air. The heater core might be getting hot coolant from the engine, but if the blend door isn’t working properly, you can still get no heat.

A stuck open blend door or faulty blend door actuator can cause the air to be

Vacuum operated blend door actuator

cold or cool.

Use a shop manual to locate the blend door actuator. Watch the actuator as you move the temperature controls to see if it’s moving the blend door. If the actuator doesn’t move, remove it from the heater box and try moving the blend door by hand. If th

An electronic blend door actuator opens and closes a blend door to regulate the air temperature in your car

e blend door moves by hand, then the problem is the actuator. Replace it.

Test the vacuum motor

If the vacuum motor is receiving vacuum but not operating the blend door, the problem may be a cracked diaphragm or a binding blend door. Remove the vacuum motor and move the blend door linkage by hand. If the blend door moves freely, replace the vacuum motor

Test an electronic blend door actuator

Locate the blend door actuator and observe the motor while adjusting the heat setting (key in RUN position).

Refer to a shop manual to locate the blend door actuator

The motor should rotate and open and close the blend door. If the motor doesn’t move, unscrew it from the heater box and try moving the blend door by hand. If the door moves easily, then the problem is in the blend door actuator or the heater control head. Consult a shop manual for the electrical tests to determine whether the motor or the control is at fault.

Check blend door actuator movement

If the door binds and cannot close properly, or the actuator fails, the door may remain open. Depending on the design of the heater box, that could cause a heat always on condition. If the motor fails when the door is closed, that could cause a no heat in car condition.

During air conditioning season, a bum blend door actuator can fail in a full or partially open position. In that case it heats the newly cooled air from the air conditioning evaporator coil, cancelling out the cooling effect of the air conditioning.

3) Check the heater control valve

Some vehicles don’t use a blend door and actuator to control cabin air temperature. Instead, they vary how much hot coolant enters the heater core by using a heater control valve. Heater control valves can be operated by a cable, vacuum motor, or electric motor.

To check the operation of a heater control valve. touch the incoming and exit heater hoses. If the valve is open, both hoses should be hot. If only one hose is hot/warm, the valve is closed. If you have a cable operated valve, try moving the lever by hand to open the valve. If you have a vacuum or electric valve, follow the diagnostic procedure shown in your shop manual.

4) A stuck open thermostat can cause a no-heat condition

The thermostat regulates the flow of coolant to the engine. If stuck open, the engine might not reach the optimal temperature, leading to insufficient heat. Conversely, if the thermostat is stuck closed, the engine could overheat. Replacing a faulty thermostat is a relatively simple and inexpensive repair that can restore heat to your car.

Here are the symptoms of a stuck open thermostat:

• The air gets warmer when you’re stopped at a stop sign. This happens because there’s no ram air going through the radiator, so the coolant stays a bit warmer. If you have warmer air when stopped and it gets colder when you drive, suspect a stuck open thermostat.
• The engine takes longer than normal to warm up. Since coolant is circulating through the radiator at all times, the engine won’t be able to heat up to its proper operating temperature as quickly. This can mean poor fuel economy and emissions performance when cold.
• The temperature gauge reads lower than normal. The driver will notice the temperature needle staying under the halfway point, sometimes near the “C” cold mark, even after the engine has been running for several minutes.

5) You may have a plugged heater core

A clogged heater core won’t allow coolant flow and you’ll get no heat in the cabin

The heater core is essentially a small radiator located behind the dashboard. Coolant flows through it, and the blower motor pushes air over the heated fins, warming the cabin. A clogged or leaking heater core can prevent heat from being generated. If you notice a sweet smell inside the car or see coolant leaking on the passenger side, these are signs of a failing heater core. Unfortunately, accessing and replacing a heater core can be labor-intensive and costly.

Heater core tip: Hot coolant (around 200°F) enters the heater core from the engine. Cabin air blows across the heater core and removes heat from the coolant. However, the exiting coolant is still hot. To check the operation of your heater core, touch both heater hoses at the firewall. Both hoses should be hot. If only one hose is hot, your heater core is likely plugged with rust or leak sealer.

6) A worn-out water pump can also cause a no heat in car condition

The water pump moves coolant through your engine and

This water pump is so worn that the impeller actually disintegrated

heater core. A water pump can be belt or chain-driven or even electric on some late-model engines. If it’s not working properly, you won’t get good coolant flow. Normally, that would result in an engine overheating warning. However, in winter, the cold air flowing past the engine may keep it cool enough to prevent the warning from showing.

A no-heat condition due to a faulty water pump is pretty rare without some other indications, but it’s not impossible.

©, 2014 Rick Muscoplat