The most common reason your car's heat stops working is low engine coolant or a faulty thermostat. Without enough coolant to carry engine heat to the heater core, or a thermostat stuck open that prevents the engine from reaching its proper operating temperature, you'll only get cold air from the vents. Other frequent culprits include air trapped in the cooling system, a clogged heater core, or a malfunctioning blend door actuator.
Here’s a breakdown of the primary causes, their symptoms, and typical repair complexities:
| Cause of No Heat | Common Symptoms | Typical Repair Complexity | Average Repair Cost Range (USD) |
|---|
| Low Coolant Level | Temperature gauge reads low, overheating possible | Low (DIY Top-off) | $10 - $20 (for coolant) |
| Faulty Thermostat | Engine warms up very slowly, poor fuel economy | Medium | $150 - $400 |
| Air in Cooling System | Heater blows hot then cold, gurgling sounds | Low to Medium (Bleeding system) | $50 - $150 |
| Clogged Heater Core | No heat, foggy film on windshield, sweet smell | High (Dashboard removal often required) | $500 - $1,200 |
| Blend Door Actuator Failure | No heat but engine temp is normal, clicking noise from dash | Medium | $200 - $500 |
A malfunctioning thermostat is a classic issue. This valve regulates coolant flow. If it's stuck open, coolant constantly circulates through the radiator, preventing the engine from warming up sufficiently to produce heat. You'll notice your engine's temperature gauge staying in the blue or low end.
A clogged heater core acts like a blocked artery. This small radiator under your dashboard is where hot coolant transfers heat to the air blown into your cabin. Over time, rust and debris can clog its tiny fins. If flushing the cooling system doesn't restore flow, replacing the heater core is a labor-intensive and costly job.
Electrical issues, like a failed blower motor resistor or the blend door actuator itself, can also be to blame. The actuator is a small motor that opens and closes flaps to direct air. When it fails, you might hear a repetitive clicking sound from behind the dashboard as it tries and fails to move.
For most drivers, checking the coolant level in the overflow reservoir (only when the engine is cool) is the first and easiest step. If it's low, top it off and look for leaks. If the level is fine, a professional diagnosis is the next best step to avoid more expensive repairs down the line.
