A heater that blows cold in January is not one problem; it is at least five, and they announce themselves differently. Work through the symptoms in order and you will usually know which part to order before the car is up on the jack.
Start with the symptom
| What you notice | Most likely cause | Where to look |
|---|---|---|
| Nothing comes out of the vents, on any speed | Blower motor or its fuse/relay | HVAC blower motors |
| Fan works only on the highest speed | Blower motor resistor or control module | HVAC parts |
| Air blows but stays cold; engine warms up normally | Heater core clogged, or blend door stuck | Heater cores |
| Air is warm at highway speed but cold at idle | Low coolant, air pocket, or weak water pump | Cooling system |
| Engine temperature gauge never reaches normal | Thermostat stuck open | Thermostats |
| Sweet smell, foggy windshield, damp carpet | Heater core leaking | Heater cores |
No airflow: blower motor and resistor
If the fan is silent on every setting, check the fuse first, then listen at the passenger footwell with the key on; a blower that hums but does not spin is seized. If the fan only runs on high, the resistor pack that steps the voltage down for lower speeds has burned out, which is common on cars that had a failing motor drawing too much current. Replace the resistor and the motor together if the motor is more than ten years old; a new resistor on a tired motor fails again within a winter.
Cold air, warm engine: heater core and blend door
The heater core is a small radiator behind the dash. Feel both hoses where they pass through the firewall with the engine warm: if one is hot and the other cool, coolant is not flowing through the core, usually because of sediment. A back-flush with a garden hose often clears it; if not, the core is replaced. If both hoses are hot and the air is still cold, the blend door that mixes hot and cold air is stuck or its actuator has failed. Actuators are listed under HVAC controls.
Heat only at speed: coolant level and air pockets
The heater core sits high in the system, so it is the first thing to run dry when coolant is low. Check the reservoir cold, top up with the correct type, and bleed the system with the heater on full; many engines have a bleed screw near the thermostat housing. If the level keeps dropping, find the leak before buying anything else: hoses, the radiator, the water pump seal and the heater core itself are the usual suspects.
Gauge never warms up: thermostat
A thermostat stuck open lets coolant circulate through the radiator all the time, so the engine and the heater never reach temperature. It is a cheap part and a short job on most engines; buy the temperature rating the manufacturer specifies, not a hotter one, and replace the gasket or O-ring with it.
Coolant type and the −37 °C mix
Use the coolant family your manual specifies (IAT, OAT or HOAT; colour is not a reliable guide) and mix it 50/50 with distilled water for protection to about −37 °C. In the coldest provinces a 60/40 mix reaches roughly −52 °C, but going richer than that reduces heat transfer. Pre-mixed jugs in the fluids collection list the protection temperature on the label.
Tools for the job
A spill-free funnel for bleeding, an infrared thermometer to compare hose temperatures, and a hose clamp plier set cover most of this diagnosis. They are in tools and equipment, and every order ships free across Canada with 30 days to return anything unused.