Every year somebody comes in on one of the first cold mornings with no heat. And every year one of the most common reasons is a thermostat that stuck open.
Last year it was me. The thermostat on my truck stuck open. Mark found it. I own a repair shop and I take care of my truck the way I tell everybody else to take care of theirs. Didn’t matter. You can maintain a vehicle perfectly and when a part goes, it goes.
So if your heat quit on you this fall it doesn’t mean you did something wrong.
Your heat comes from your engine
There’s no separate heater in your car like the space heater under your desk. The heat is leftover from the engine. Coolant (most people call it antifreeze) runs through the engine and picks up heat. Some of that hot coolant goes to a little radiator behind your dashboard called the heater core and the fan blows air across it. That’s the warm air on your feet.
So when the heat goes cold either hot coolant isn’t getting there, the engine isn’t getting hot enough to begin with or the warm air is getting sent the wrong way.
The thermostat, like mine
The thermostat is a valve that keeps coolant in the engine until the engine warms up. Then it opens. When one sticks open the engine never gets up to temperature. Your temperature gauge sits lower than it normally does and the heat comes out lukewarm at best. It’s usually worse on the highway with all that cold air coming through the front of the car.
It won’t leave you on the side of the road but it’s not something to just live with either. An engine that runs cold burns more gas and it can turn your check engine light on. It’s usually one of the easier no-heat fixes.
Low coolant is the one you don’t ignore
This is the one I worry about. That same coolant is what keeps your engine from overheating. The heater core sits up high so when coolant gets low it’s usually the first thing to lose out. A lot of times the heat is fine going down the road and goes cold sitting at a light. That’s a pretty good sign you’re low.
Your car doesn’t use up coolant like it uses gas. If it’s low it’s leaking somewhere. Topping it off gets you by for a little while but somebody needs to find the leak.
If your heat goes cold and the gauge is heading toward hot or you see steam, pull over and shut it off. Nobody wants to wait on a tow. A tow is still a lot cheaper than an engine.
The rest of the list
If you’ve had coolant work done or you’ve been topping off a leak for a while, air can get trapped in the system. You’ll usually hear gurgling behind the dash. That air has to be bled out right or the heat keeps acting up.
The heater core itself can plug up or start leaking. Coolant wears out and if it never gets changed it leaves gunk behind and the passages in a heater core are tiny. A leaking one gives you a sweet smell inside the car, a greasy film on the windows or a wet carpet on the passenger side. On a lot of vehicles the heater core is buried behind the dash and getting to it is a big job. Changing your coolant when it’s due does a lot to keep this one from happening.
Coolant is poisonous and it tastes sweet to dogs and cats. If you see a puddle in the driveway clean it up.
There’s also a little door behind the dash that mixes the hot and cold air. If it gets stuck you get cold air even though everything else is fine. Clicking behind the dash when you change the temperature is a giveaway. So is one side of the car blowing hot and the other side blowing cold.
And if no air comes out at all or the fan only works on high, that’s the fan and not the heat. Different problem.
If you want to look before you call us, wait until the engine is cold and pop the hood. Find the plastic coolant jug with the lines on the side. If it’s under the low line that’s your first clue. Never open the radiator cap on a hot engine, it’s under pressure and it will burn you.
When you bring it in tell us when the heat goes cold. At a stoplight, on the highway, one side only. That helps us find it.
We’ll find out why before we tell you what it needs. If it’s a thermostat, it’s a thermostat. Mine was.
Heat blowing cold? Bring it in
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About the Author
Jon Bockman has owned Bockman’s Auto, Truck and Tire since 1999, continuing what his father Charlie started in DeKalb County in 1964. Named NAPA Shop of the Year out of more than 18,000 centers, and voted Best Auto Repair in the Daily Chronicle Readers’ Choice 17 years in a row. Two locations, 24 employees, and one goal: treat every customer like a neighbor.