A car heater that blows cold air usually has a coolant flow problem, a stuck thermostat, or a failed blend door. Here are the most likely causes ranked by how often they turn out to be the problem.
The heater core needs hot coolant flowing through it. Low coolant means air pockets in the core and cold air at the vents.
A thermostat stuck open never lets the engine reach operating temp. The coolant gauge stays low and the heater blows lukewarm.
Years of corrosion plug the tiny passages in the heater core. Both heater hoses are hot but no heat comes through the vents.
A failed blend door motor stuck on cold makes the heater blow cold even with a working system. Often paired with clicking behind the dash.
A weak water pump impeller does not push enough coolant through the heater core. Often paired with overheating.
The car is overheating, you see coolant under the dash on the passenger floor (leaking heater core), or the windshield fogs up with sweet-smelling steam. A leaking heater core dumps coolant inside the cabin and can leave you stranded.
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Most often low coolant or a stuck-open thermostat. Both prevent hot coolant from reaching the heater core.
Thermostat: $150-$300. Heater core flush: $100-$200. Heater core replacement: $600-$2000. Blend door actuator: $200-$700.
Yes. A weak impeller does not push enough coolant through the heater core. Often paired with the engine running hot in stop-and-go traffic.
Disconnect both heater hoses and back-flush with a garden hose. If brown crud comes out and heat returns, you saved a $1000+ repair.
Higher engine RPM pumps more coolant through the heater core, masking a partial clog or weak water pump. At idle the problem shows itself.