Car Heater Maintenance

Car Heater Maintenance

Diagnosing your vehicle's heater problems

The eternal optimist in all of us anticipates that a simple flip of the heater switch on that first really cold, blustery day of winter means that our vehicle will instantly be as warm and cozy as the toasty home we just left.

How Heaters Work

Get real. Granted, most vehicle systems do work the way they're supposed to; but, that first day when the world is coated in frost and the temperature has plummeted below freezing is not the time to find out your car's heater is not working. Start by flipping on that heater switch well before you really need the heat. If your heater doesn't respond with a warm blast of air, here are a few things you can do to figure out the problem.

Only once the engine warms up will the passenger compartment warm up as well.

Automotive heaters work off the warm air absorbed by the radiator from the warmed-up engine. If your cold-day car ritual is to turn on the heater and blower right after your turn the ignition switch, all you'll get is cold air. Only once the engine warms up will the passenger compartment warm up as well.


Your radiator has to have an adequate level of coolant for the heating system to work. If you're low on coolant, air pockets can develop in the heater core and hoses, blocking the flow of warmth. To make sure the coolant is fully circulating, check both heater hoses (inlet and outlet return hoses) when the engine has reached normal operating temperatures and the heater is on. Both hoses should be hot to the touch. If they're not, you may have identified the source of your problem.

For a front-wheel drive or rear-engine configuration, you may want to take the problem to your mechanic, since those systems are usually a bit more complicated. Filling the cooling system and bleeding out heater hose air pockets may require opening up bleeder valves on the thermostat housing or hose connections. If your vehicle doesn't have these valves, you may have to loosen the heater outlet hose as you add coolant to the radiator. In either case, leave the valve or hose open until the fluid starts to dribble out. At that point, the air pockets should be eliminated and you'll be good to go.

A heater core clogged with cooling system crud can also cause the heater to malfunction.

Another source of no-heat is a thermostat stuck in the open position. A good test is to start a cold engine and check the upper radiator hose. No coolant should be moving through the hose until the engine starts to warm up, and at that point, the hose will feel warm. If the thermostat is stuck on open, you'll feel the coolant in the hose as soon as the cold engine is started.

A heater core clogged with cooling system crud can also cause the heater to malfunction. The only solution is a new core. The way to avoid this repair is to flush the cooling system as part of your regular maintenance schedule.

If both your heater and defroster are on the fritz, the problem is probably an inoperative airflow control or inlet door in the heater ducting. The quick fix is to check for and repair or re-attach a loose cable or hose. If your vehicle has a vacuum-actuated system, you'll have to replace the vacuum motor or control switch.

What if your heater is working well, but the blower that's supposed to get the warmth into the passenger compartment isn't working? This is easy enough to figure out. You should be able to hear the blower blowing air when you flip it on. If it's silent, that's the problem. This may be as simple as a blown fuse or as complicated as a broken motor or wiring/electrical problem.

One word of caution: if you've checked your fuse box diagram and can see that the fuse has blown, simply replacing the fuse may not solve the problem. Fuses blow when the circuit overloads, so you or your mechanic should identify the cause of the circuit overload. If a replacement fuse blows out immediately, there's a short in the system. If the fuse lasts a while, then goes bad, the fan motor may be running too hot due to worn brushes.

No heat can also be caused by pinched heater hoses, which you should be able to locate and un-pinch. Another potential source of a malfunctioning heater is a weak water pump that isn't properly circulating the coolant through the system, or a cooling or clutch fan that's over-cooling your engine.

One thing you should NOT do is place a barrier, such as a piece of cardboard, in front of your vehicle radiator. This is a common "don't-wanna-figure-out-the-problem" solution to increase heating capacity. Unfortunately, the result may be an overheated engine, even in the coldest weather, especially when the engine is under load or kept idling in traffic.

As with most mechanical problems, the solution to inadequate heating can be simple or complicated. Even with an easy fix, it's better to take care of that before you're caught out in the cold.

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