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Car heater not blowing hot: what it could be
The heater uses the heat of the engine coolant, so if it blows cold air the coolant is often low, the thermostat is stuck open and the engine does not warm up, or the blend door is at fault. On very efficient cars it is normal for it to take a little longer in winter.
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Reviewed by Daniel Morales, founder of Reprorace SL, on
Is it serious? Can I keep driving?
Usually not urgent, but do not ignore it. If the engine temperature is normal you can drive, but without heat you may not be able to clear the windows: do not drive with fogged glass. If the temperature gauge climbs or the coolant level drops, have it checked as soon as possible.
A careful, general answer: only someone who inspects the car can say whether it is safe to drive. When in doubt, stop somewhere safe and call for assistance.
Most likely causes
- Low coolant or air in the system: the heater core does not get hot coolant.
- Thermostat stuck open: the engine takes a long time to warm up and the gauge stays low.
- Blend door or temperature control: the air does not pass through the heater core.
- Clogged heater core: heats poorly even with a hot engine.
- Water pump: less common: low coolant flow.
Listed from more to less common in general; on your car they may be different. A fault-code reading and a check at the garage confirm which one it is.
To understand it better: What does the engine thermostat do and what does P0128 mean? What is live data and what is it for?
Car system: Cooling
What you can check yourself
See whether the engine temperature gauge reaches its normal position after a few miles.
With the engine cold, check the coolant level.
Move the temperature control from cold to hot and listen for the blend door moving.
Safe checks only: with the engine off and cold whenever you look under the hood, without taking anything apart and without touching hot or moving parts.
Related fault codes
- P0128Coolant thermostat: coolant temperature below the thermostat regulating temperature
- P0597Thermostat heater, control circuit open
- P2560Engine coolant level: too low
- U0164Lost communication with HVAC control module
Generic codes that often come with this symptom. Seeing one does not confirm the cause: it is where the diagnosis starts.
How the ECO-LITE helps diagnose it
Plugged into the OBD socket, the ECO-LITE reads the fault codes of the control units that answer, with the freeze-frame data captured when the fault happened, and shows live data on your phone or computer. With your permission, your garage can see them remotely before you bring the car in.
What to look at here: live coolant temperature and how long it takes to rise; code P0128 means the engine does not reach the expected temperature.
Frequently asked questions
Why does it take so long to warm up in winter?
Efficient engines produce less heat. If the gauge reaches its spot and then the heat comes, it is normal.
Does a stuck-open thermostat use more fuel?
Yes: the engine runs cold for longer and the control unit enriches the mixture.
Can I drive without a heater?
Yes, if you can keep the windows clear; the A/C helps dry them.
Keep reading
- Engine cooling: how it works and what fails
- Low coolant warning light on: what to do
- Engine runs rough when cold and fine when warm: what it could be
- Car A/C not blowing cold: what it could be
- Car symptoms: what they mean and what to do
- Check engine light on: what to do
- How to read and clear your car's fault codes
- OBD fault codes: what each one means