Symptom guide
Heater Blowing Cold Air
Why a car heater may blow cold even when the engine is running.
Quick answer
Cold heater air can come from low coolant, thermostat problems, heater core blockage, blend doors, or trapped air.
Most likely causes
- Low coolant
- Stuck-open thermostat
- Air in cooling system
- Heater core blockage
- Blend door actuator fault
Basic driver checks
- Watch engine temperature gauge.
- Check coolant level cold.
- Notice if heat improves while driving.
- Compare left and right vent temperature.
DIY diagnostic path
- Do not open hot cooling system.
- Check for coolant smell inside cabin.
- Look for wet passenger floor.
- Scan for P0128 if warm-up is slow.
Common mistakes
- Replacing blower motor when airflow is strong but cold
- Ignoring low coolant
- Assuming thermostat without checking level
What to tell the mechanic
Say whether engine reaches normal temperature and whether coolant level drops.