My heater blows cold air

The engine is warm but the heat stays cool or lukewarm. The heater uses hot engine coolant, so it often starts there.

Get to it this week

A starting point, not a diagnosis. Most parts stores read check-engine codes and test batteries for free.

Likely causes — tick the parts you want

Low coolant
most likely · Most people can do this
How to check: With the engine completely cool, check the coolant overflow tank. Below the MIN line means it is low and there may be a leak somewhere.
- Antifreeze/coolant (type for this car)(Coolant types differ — ask the counter to match the type for this car.)
A thermostat stuck open
possible · Doable with tools and a free afternoon
How to check: If the temperature gauge takes a long time to reach the middle or sits lower than it used to, the thermostat may be stuck open.
- Thermostat(Ask whether this car’s thermostat comes with a new gasket or housing.)
- Antifreeze/coolant (type for this car)
A clogged heater core or stuck blend door
less likely · A mechanic’s job
How to check: If coolant is full and the engine warms normally, but the heat still blows cold or only on one side, a mechanic should check the heater core and blend door.

Be careful

Never open the radiator cap or coolant reservoir on a hot engine — the coolant is under pressure and can scald you. Wait until the engine is cool to the touch. Coolant is poisonous and tastes sweet to pets, so wipe up every spill.

Your car — the counter needs this

YearMakeModelEngine (optional)

VIN (optional, most exact)Which parts store (optional)

If the counter doesn’t have a part, your shopper asks them to order it under your name and messages you the order number. When it comes in, send someone to collect it.

Not quite it?

My A/C blows warm air My vents smell musty