Jeff's DIY

Advice on doing your own diagnosis and repair

Advice from the book Can I Do It Myself?

Heating and Air Conditioning


No Heat? Fan Blowing Cold Air?

There are a few reasons this might happen to your system. A typical system is shown in the diagram below:

HVAC system diagram


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


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:

Heater control valve


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:

Coolant bleed ports


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 your 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.



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. To replace the thermostat, see my tip on that subject, but ignore the discussion on overheating as that tip assumes the thermostat has failed closed.


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.


Typical blend door actuator


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, I 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.


Why Does My Heater Fog the Windshield?


Water vapor on the windshield or coolant smell in the car indicates a leak in the heater core.  A small leak may only fog the glass, but a larger one will leak on the floorboard.  The leak may be temporarily fixed by disconnecting the heater hoses from the core at the firewall (in the engine compartment) and looping one of the heater hoses between the high and low pressure ports of the engine.  This will disable the heater until the core can be replaced.


Why Doesn't My Heater or Air Conditioner Blow Air?


The following components are needed for a typical blower circuit:

1. Power from the battery or alternator, wiring thru a fuse and possibly an accessory relay to the switch
2. Switch and wiring to channel voltage to the appropriate resistor corresponding to the desired speed
3. Resistor set or blower speed module to reduce voltage to the blower for settings less than high
4. Final wiring to blower

5. Blower

Since designs vary greatly among auto makers and system components can be difficult to find, we recommend you access a manual for your vehicle to determine the specific system design. Manuals can often be found at libraries or online. offers a significant library of free repair information at their website.

Because there is typically no relay between the switch and the resistor, all of the blower current runs through the switch. This sometimes causes the switch to overheat and fail over time. Depending on how the switch is designed, the failure may allow multiple currents to run to the resistor pack, and this may in turn cause the resistor pack to overheat and fail. An example of the circuit I'm describing is shown below:

Typical blower motor circuit


This is a typical GM design, and you should obtain the wiring diagram for your particular blower circuit, if possible, to help you troubleshoot your circuit. Note that if the above switch was to contact two terminals simultaneously, there would be more current than the resistor pack is designed for. This can burn the resistors and/or blow the thermal breaker. Also note that if the resistor pack fails, the blower can still run at full speed. This design even uses a separate relay for the high speed to avoid sending that current through the control switch. Newer models may use a blower motor control module. In these systems, there is no separate resistor set, but there is usually a thermal fuse on the module that is a common failure point. In some designs there is a relay that must close for the blower to work at any setting. This then becomes an additional failure point to troubleshoot.


Blower works on high only:

If your blower is working only on high speed, the likely cause is a failed resistor pack or thermal fuse. Always check fuses first, because they are easy and inexpensive to replace. In some designs, such as the one in the wiring diagram above, there is a separate fuse for the lower speeds.

To verify a failed resistor pack, locate and remove the resistors. The resistor pack is installed in the air stream of the system to help cool the resistors. It is usually accessible from under the dash on the passenger's side or sometimes from the engine compartment along the firewall. In some cases, it is even attached to the blower motor housing. It is identifiable by the connector with several wires coming out-some going to the switch and some to the blower motor. You may have to remove a trim panel under the dash or the glovebox to access the resistor. A typical installation looks like the picture below:

Installed blower resistor


Remove the connector and unscrew the resistor. Remove it and inspect for heat damage. If you have an ohmmeter, check each resistor for open and shorts. Replace as appropriate.

If your resistor failed, the system will likely work again after replacing the resistor pack; however, if it failed because of a faulty switch, the resistor will likely fail again. For this reason, we always recommend checking the switch for heat damage and proper operation after a resistor pack has overheated. See procedures near the end of this tip for troubleshooting the switch.

Blower does not work at all:

If the blower does not work at all, odds are that the problem is the fuse, relay, or the motor itself. First as always, check the fuse. Blower fuses are most often found in the engine compartment fuse box but may also be found in the dash fuse box.

The blower motor is usually accessible from above the passenger's feet, but in some models it comes out from the engine compartment. You may need to remove a panel to see where the motor is mounted. Consult a manual on your vehicle for the easiest method of accessing the blower motor.

Once you have found the blower, remove the connector and interrogate with a test light or voltmeter. Turn the key on and check the voltage at the blower. There should be 12 volts when the switch is on high. Each other speed setting should indicate a lower voltage. If there is no voltage (assuming fuses are good), suspect your switch or resistor pack has failed. If voltage is present, the motor is likely bad. In this latter case, remove the motor and connect it directly to your battery to verify it does not work. Replace as appropriate.

Blower switch testing:

You need an ohmmeter to test a switch, but if you have a power supply and test light, you can do an improvised test. Assuming you have a typical design, testing the switch consists of verifying that for each setting there is one and only one resistor terminal that is connected to the source terminal. If there are opens or multiple connections, replace the switch. If your system is not of the typical design shown in the wiring diagram at the beginning of this tip, consult your specific diagram for appropriate test procedures.


Auto/Truck A/C Recharge

If your A/C system is low on refrigerant, recharge is a DIY-able task. Be aware that your system is leaking and may not hold a recharge very long. Also, if the system is too low for the compressor to run, it may not recharge by this method.

Below is a diagram of a typical system that I copied from

A/C system diagram


For more details on system operation, see
Typical leaks occur in the evaporator, condenser or compressor. A good shop can isolate and replace the leaking component. There are still two types of auto A/C systems out there. The older systems use R-12, a.k.a. Freon. Freon is no longer available on the open market, but I believe licensed A/C mechanics can get it for a high price. All newer cars use R-134a, which is not as effective but is more environmentally friendly. You can buy R-134a in any parts store or even at WALMART. You will also need a hose to recharge your system. Buy at the same location or borrow from a friend.

If you have the old R-12 system (indicated on the compressor tag), you will need a conversion kit to make your system run on R-134a. Even if you convert your system, you may or may not be happy with the performance of R-134a in a system that was designed for R-12.

Back now to recharge: the recharge hose uses a quick-disconnect fitting and will only fit on the low pressure port. The low pressure port is somewhere between the evaporator (inside the car) and the compressor (attached to the side of the engine near the front (end with the belt on it). Trace the aluminum pipes coming out of the engine compartment firewall (usually on the left side of the firewall as you look in from the front of the car) until you find the port on the one that goes to the compressor. Sometimes the dryer is on the low side and the port may be on the dryer. Here is a picture of a case wherein the low pressure port is on the dryer. The refill hose is attached via the quick disconnect fitting.

Recharging system


(Note, the high pressure port is on the line going from the radiator area back to the compressor and is a larger port.) Remove the cap from the port, pull back the ring on your hose QD fitting, shove it onto the port and release the ring--it should seat without leaking. Next, attach the can of R-134a to the other end of your hose (some of these have the can screw in and others use a cam to lock it in), close the valve on the hose (clockwise) all the way. It should pierce the can and open a hole to release the R-134. Now open the valve fully. The R-134a should not leak if you have the can seated in the hose fitting properly. Note, if you live in California, R-134a cans use a rubber valve in the top of the can that seals when the hose is removed.

Now start the car and turn the A/C on max. You are now recharging your system. Jiggle the can without putting tension on the QD connection. Follow directions on the can. If there is oil or sealant mixed in the R-134a, the can may instruct you to turn the can sideways occasionally to get everything into the system.

Note, some hoses come with a gage that you can use to determine when the system is full. I usually just go by how cool the air is and by the formation of frost on the low pressure line. If the can empties and the air is not as cold as you are used to getting from the system, put another can on. If you cannot empty the can after say 20 minutes, the system is probably full. In this case, close the valve. In either case, once your system is full or you are happy with the performance, disconnect the QD and replace the cap on the port. If you have R-134a left over, use it in another car. If you just leave it sitting, it will eventually leak out of the can thru the valve or at the fitting to the hose.

Now take the car for a spin and enjoy the cool air.

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