So, Mother Nature has graced us with some cold weather again. It’s that time of year where everyone finds out if their car still heats the interior or not, and everyone goes into panic mode trying to figure out what’s wrong if it’s not working properly.

Typically when a customer says, “My car has no heat,” it means one of two things:

  1. Air is coming through the vents, but it’s not warm.
  2. No air is coming through the vents at all

Your engine gets hot. Coolant keeps the engine at a regulated temperature, but the fluid obviously gets hot from passing through a hot engine. This coolant is diverted through what’s called a heater core. This is the source of your heat.

This little guy looks like a radiator. Fins attached to the core tubes serve to increase surface for heat transfer to air that is forced past them, by a fan (blower motor), thereby heating the passenger compartment.

Typically, if you have no air coming through at all, this is your culprit. It is an electric motor, so it can and will fail. What if you can hear the motor, but can’t feel any air coming out? The “squirrel cage” could be broken. Pretty much a fan without any fan blades at this point. It can’t draw any air through if the fins are broken.

If the air coming in is cold, there are a few things it could be.

Inside your dash, there’s a series of ducts and doors to divert air to different places and different ways. In some vehicles, these doors (called blend doors) are moved by a small actuator, which is basically a tiny motor. In other vehicles, these doors are moved by cables/rods attached to your controls.

Most vehicles with digital or automatic HVAC (Heating Ventilation & Air Conditioning) systems use actuators, while manual controls use the cables/rods.

So, it’s possible that you’ve got an electronic problem like the situation with the rear HVAC of my ’05 Yukon Denali (inside temperature sensor/actuator/module), or maybe a mechanical problem (blend door broken or stuck). In my case, I’ve replaced the temp door actuator, control module, and relearned the system to no avail. I can’t figure mine out, so I manually set my temp door in the spring to stay on cold, and in the fall to stay on hot.

Another possibility if the air is coming in cold, is a plugged heater core or a faulty thermostat. If the engine doesn’t get to proper operating temperature, the coolant will never get warm enough which would indicate a faulty thermostat. If the heater core becomes plugged, coolant can’t flow through it and warm it up.

You used to see this a lot in the 70’s, 80’s, and early 90’s. People did stuff like this during the winter to stop the frigid outside air from cooling down the radiator too much, which lowered the temperature of the coolant. Not advisable today, as not enough outside air flow through the radiator can cause your engine to overheat and cause serious damage to the engine.

My ’98 Bonneville SE. My blower motor (fan) has 4 speeds. 1 doesn’t work, 2 is low, 3 is high, 4 doesn’t have any change from 3. What does all this mean (usually)?

Blower motor resistor. This thing does exactly what its name implies. An electric motor needs a component to regulate its speed. Typically when these fail, you’ll experience a situation similar to mine, or maybe it only works on the highest setting because the resistor is not resisting anymore.

Edit: The key word there was, “Usually.” I replaced mine since this posting and it didn’t fix it. Proper diagnosis, it comes back to a lack of this. This car has a blower motor high speed relay. A relay that specifically controls the “high” setting on the blower switch. Evidently mine is bad. Neat.

First, you have to know if the cause is engine related. Check your temperature gauge. It should read about midway (think half-tank on your fuel gauge), or near 200º F. If it’s barely climbing, a thermostat stuck open is likely the culprit.

Check your heater hoses that run into the firewall that attach to the heater core. Both of them should be warm/hot to the touch when the engine is hot. If they are, and you still have no/weak heat inside, you could have a plugged/partially blocked heater core. If this is the case, have your heater core AND your cooling system flushed out.

Hopefully none of these problems plague your vehicle when you need it most, but if they do, come see your trusted auto repair technicians here at Auto Works.