There are a few reasons this might happen to your system. A typical system is shown in the diagram below:
The system requires hot water from the engine to pass through the heater core and the blend (or mix) door positioned for air pushed by the blower motor to pass through the core. If the vehicle has no air conditioning, the evaporator is absent from the system. Some designs also incorporate a control valve in the heater hose to stop coolant flow to the heater core when not in use. The blend door position is actuated by a stepper motor or cable.
Typical failure modes are as follows:
1. Blend door stuck
2. Blend door actuator failed motor or stripped gears
3. Blend door cable is broke or needs adjustment
4. Temperature control switch failed
5. Air pockets in engine cooling system
6. Clogged heater core
7. Heater coolant control valve failed closed
8. Thermostat failed open
Troubleshooting:
First, with your engine hot and the heater turned on, feel the heater hoses in the engine compartment as they attach to the heater core at the firewall. If either of the hoses is cold, the problem is likely a lack of adequate coolant flow.
No or low coolant flow:
Check for the presence of a control valve in one of the heater hoses--trace the hoses all the way to the engine. Not all vehicles have such valves. If you find one, check to see if it is actuating properly. It should receive a signal of some kind from the heater controls and open when the temperature control is turned to heat. If you cannot determine that the valve is opening, try taking one of the clamps off and removing the hose to see inside the valve. A typical vacuum operated heater flow control valve is shown below:
The valve may also be actuated by a solenoid or wire cable.
If you have no valve, next check the cooling system for air. If you have a radiator cap, take it off and look inside. If it is not full, fill it with coolant and start the engine. Turn the heater on and let the engine run until it is warm and the thermostat has opened. Continue to fill the radiator or reservoir until the level stops falling. Check for the presence of any bleed ports in the system. Bleed ports may be on the housings that radiator or heater hoses attach to or may be installed in the hoses themselves. A typical Honda application is shown below:
For stubborn cases, park the car uphill, take cap off radiator, start car, fill radiator, let it warm up until thermostat opens, bleed air out, fill to the neck of the radiator and watch for bubbles coming up. Once you have bled the air, if gas continues to come out, you may have a blown head gasket that is forming bubbles that can block coolant from entering the heater. In these cases, the heater may work well at speed but blow cold at idle.
Clogged core: If the control valve is open and there is no air in the system, your heater core may be clogged. A quick way to check for a clogged core is to turn the blower motor off for a while and then back on. If it produces warm air for only a short time and then cools down, your core is only passing a small amount of coolant. This can also be caused by a bad control valve, if you car has one or possibly by a weak water pump. You can try flushing the core by removing the heater hoses and attaching a garden hose or other source of water pressure to force water through the core. Use an adapter to make a good seal and connect to the lower pipe to backflush the core. If the core will not flush or allow adequate flow, replace the heater core.
Thermostat: if your car temperature is running cold and only warms up when idling, the heater will do the same. This is caused by a thermostat that has failed open, allowing too much coolant to flow through the radiator.
Blend door not moving to heat position:
If you have coolant flow through the heater core, the problem is with the blend door. The blend door is almost always actuated from under the dashboard. Many systems use an electric motor to actuate the blend door. Check any heater or A/C fuses before tearing into your system. You may need to remove a partition to see the actuator. The door should be to the right of center under the dash. A typical electric motor actuator design is shown below.
The actuator can be on the top front or bottom of the duct. Consult a manual for your vehicle if you cannot locate the actuator. Once you can see the actuator, watch it while you change the temperature setting (key on unless it is cable operated). If the actuator doesn't move or turn, troubleshoot the reason.Systems such as the one in the above picture typically fail in the shaft attach or the internal gears on the actuator. If your actuator is getting power but not moving the door, replace the actuator. If the actuator is not getting power, troubleshoot the control switch. Due to the wide variety of designs, we cannot provide test tips for electric blend door control switches.
Whether your blend door is electric or cable operated, remove the actuator and operate the door with your hand. If the door sticks, the heater box must be removed and opened to free the door and restore unhindered operation.
If your blend door is cable operated, check the cable for proper attachment at the control lever and at the door lever. The cable sheath must be properly adjusted and firmly clamped for proper operation. To adjust, first remove the cable from the door and move the door lever with your hand to observe the range of motion. Then set the control to full hot, open the door fully, and reattach the cable, clamping the sheath so as to ensure the door opens fully when set to hot.
The system requires hot water from the engine to pass through the heater core and the blend (or mix) door positioned for air pushed by the blower motor to pass through the core. If the vehicle has no air conditioning, the evaporator is absent from the system. Some designs also incorporate a control valve in the heater hose to stop coolant flow to the heater core when not in use. The blend door position is actuated by a stepper motor or cable.
Typical failure modes are as follows:
1. Blend door stuck
2. Blend door actuator failed motor or stripped gears
3. Blend door cable is broke or needs adjustment
4. Temperature control switch failed
5. Air pockets in engine cooling system
6. Clogged heater core
7. Heater coolant control valve failed closed
8. Thermostat failed open
Troubleshooting:
First, with your engine hot and the heater turned on, feel the heater hoses in the engine compartment as they attach to the heater core at the firewall. If either of the hoses is cold, the problem is likely a lack of adequate coolant flow.
No or low coolant flow:
Check for the presence of a control valve in one of the heater hoses--trace the hoses all the way to the engine. Not all vehicles have such valves. If you find one, check to see if it is actuating properly. It should receive a signal of some kind from the heater controls and open when the temperature control is turned to heat. If you cannot determine that the valve is opening, try taking one of the clamps off and removing the hose to see inside the valve. A typical vacuum operated heater flow control valve is shown below:
The valve may also be actuated by a solenoid or wire cable.
If you have no valve, next check the cooling system for air. If you have a radiator cap, take it off and look inside. If it is not full, fill it with coolant and start the engine. Turn the heater on and let the engine run until it is warm and the thermostat has opened. Continue to fill the radiator or reservoir until the level stops falling. Check for the presence of any bleed ports in the system. Bleed ports may be on the housings that radiator or heater hoses attach to or may be installed in the hoses themselves. A typical Honda application is shown below:
For stubborn cases, park the car uphill, take cap off radiator, start car, fill radiator, let it warm up until thermostat opens, bleed air out, fill to the neck of the radiator and watch for bubbles coming up. Once you have bled the air, if gas continues to come out, you may have a blown head gasket that is forming bubbles that can block coolant from entering the heater. In these cases, the heater may work well at speed but blow cold at idle.
Clogged core: If the control valve is open and there is no air in the system, your heater core may be clogged. A quick way to check for a clogged core is to turn the blower motor off for a while and then back on. If it produces warm air for only a short time and then cools down, your core is only passing a small amount of coolant. This can also be caused by a bad control valve, if you car has one or possibly by a weak water pump. You can try flushing the core by removing the heater hoses and attaching a garden hose or other source of water pressure to force water through the core. Use an adapter to make a good seal and connect to the lower pipe to backflush the core. If the core will not flush or allow adequate flow, replace the heater core.
Thermostat: if your car temperature is running cold and only warms up when idling, the heater will do the same. This is caused by a thermostat that has failed open, allowing too much coolant to flow through the radiator.
Blend door not moving to heat position:
If you have coolant flow through the heater core, the problem is with the blend door. The blend door is almost always actuated from under the dashboard. Many systems use an electric motor to actuate the blend door. Check any heater or A/C fuses before tearing into your system. You may need to remove a partition to see the actuator. The door should be to the right of center under the dash. A typical electric motor actuator design is shown below.
The actuator can be on the top front or bottom of the duct. Consult a manual for your vehicle if you cannot locate the actuator. Once you can see the actuator, watch it while you change the temperature setting (key on unless it is cable operated). If the actuator doesn't move or turn, troubleshoot the reason.Systems such as the one in the above picture typically fail in the shaft attach or the internal gears on the actuator. If your actuator is getting power but not moving the door, replace the actuator. If the actuator is not getting power, troubleshoot the control switch. Due to the wide variety of designs, we cannot provide test tips for electric blend door control switches.
Whether your blend door is electric or cable operated, remove the actuator and operate the door with your hand. If the door sticks, the heater box must be removed and opened to free the door and restore unhindered operation.
If your blend door is cable operated, check the cable for proper attachment at the control lever and at the door lever. The cable sheath must be properly adjusted and firmly clamped for proper operation. To adjust, first remove the cable from the door and move the door lever with your hand to observe the range of motion. Then set the control to full hot, open the door fully, and reattach the cable, clamping the sheath so as to ensure the door opens fully when set to hot.
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