A car heater blowing cold air usually means hot coolant is not reaching the heater core, or the air is not passing through it. The common causes are low coolant or an air pocket, a thermostat stuck open so the engine never warms up, a clogged heater core, and a failed blend door actuator that keeps the temperature door on cold. If the heater blows cold and the engine is overheating, low coolant is the likely cause and needs attention right away.
Causes
| Cause | Clues |
|---|---|
| Low coolant or air in the system | Heat comes and goes, gurgling behind the dash; see how to bleed coolant |
| Thermostat stuck open | Temperature gauge reads low, slow warm-up, code P0128 |
| Clogged heater core | Both heater hoses not equally hot; weak heat at idle |
| Blend door actuator | Clicking behind the dash, one side hot and one cold |
| Weak water pump | Heat drops at idle, improves with rpm; see water pump symptoms |
Heater blowing cold air and the engine overheating
When both happen together, the engine is usually low on coolant, so there is not enough coolant to reach the heater core and cool the engine. Stop and let it cool before checking the level. If coolant keeps disappearing, see losing coolant but no leak and signs of a blown head gasket.
Cold air with no coolant leak
With full coolant and no leaks, look at the thermostat, the blend door and the heater core. Feel both heater hoses at the firewall once the engine is warm: if both are hot, the problem is the blend door or airflow; if one is cooler, the core is restricted or coolant flow is weak.
Cold air on one side
On dual-zone systems, one side blowing cold usually means one blend door actuator has failed. On some vehicles, low coolant also shows up first on one side.
Cold air when idling, warm when driving
This points to low coolant, air in the system or a weak water pump, since higher rpm pushes more coolant through the core.
Heat that fades while driving
Heat that starts warm and fades on the highway in winter points to a thermostat stuck open: the cold air through the radiator overcools the engine, and the gauge drops below its normal position. Heat that fades and comes back with gurgling noises points to air or low coolant.
Fixes
- Top up with the correct coolant and bleed the air out.
- Replace a thermostat that is stuck open; check the coolant temperature sensor reading.
- Flush or replace a clogged heater core.
- Recalibrate or replace a blend door actuator.
Related
For overheating causes see engine overheating; for overheating only in traffic see car overheating when idling.
More guides: thermostat stuck open.
Trouble codes: P0125 Insufficient Coolant Temp for Closed Loop · all OBD2 codes.
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