Heat comes from the engine’s coolant
A car heater does not make heat of its own. Hot coolant from the engine flows through a small radiator behind the dash called the heater core, and the blower pushes air across it. A door inside the heater box, the blend door, decides how much of that air goes through the core and how much bypasses it.
That means a cold heater usually has one of a handful of causes:
- Coolant that is low, or has an air pocket trapped in the heater core after a repair.
- A thermostat stuck open, so the engine never reaches full temperature. The gauge sits low and the heat fades on the 417.
- A heater core partly blocked with scale or old coolant.
- A blend door actuator that has failed, often with a clicking from behind the dash, or heat on one side only.
The first two are cooling system faults, so the cooling system page goes into them in more detail. In Ottawa, a car with no heat in January is also a car that cannot keep its windshield clear, which makes it a safety problem rather than a comfort one.
A fan that only runs on high
This is one of the most common heating complaints. On many cars the lower fan speeds pass through a blower resistor that drops the voltage, and the high setting skips it. When the resistor burns out, high is the only speed left. On some cars it is an electronic blower module instead.
If the fan does not run at all, the blower motor, its fuse or relay, or the control panel is suspect. A squealing or ticking fan often has leaves or debris in the housing. The cabin air filter is worth a look too, since a clogged one cuts airflow on every setting. Wiring faults behind the dash are tested the same way as any other circuit on our electrical repair page.
Windows that fog and will not clear
On most cars, selecting defrost runs the A/C compressor even in winter. The A/C dries the air before it is heated, which is what clears fog from the glass. So a car with a dead A/C can be slow to defog in November, even with plenty of heat.
Fogging also comes from moisture in the cabin. Snow melting off boots soaks the carpets, and recirculate mode traps that damp air inside. A greasy film on the inside of the glass with a sweet smell is a heater core leaking coolant, and it should be checked rather than wiped away every morning.
A/C that blows warm
An A/C system is sealed. If it is low on refrigerant, it is leaking somewhere, and simply adding more only buys time. The condenser sits at the very front of the car, ahead of the radiator, where it takes stones from the road and a winter’s worth of salt spray. Corroded condensers and leaking hose fittings are common here.
Other causes have nothing to do with refrigerant. A compressor clutch with too wide a gap, a failed pressure switch, a blown fuse or a cooling fan that never comes on can all leave you with warm air. Testing pressures and the electrical side together tells which one you have, and you get the price before anything is opened up.
Newer cars use a different refrigerant called R1234yf, and older ones use R134a. The two are not interchangeable, and each needs its own service equipment.
Smells from the vents
A musty smell when the A/C first comes on is usually mould on the evaporator, often because its drain tube is blocked. A blocked drain can also leave water on the passenger floor. A sweet smell points at coolant. A hot electrical smell from the vents, especially on the lower fan speeds, means the blower resistor or its connector is overheating and should be looked at soon.
Check it before the season turns
The best time to find a weak heater is October, before the first cold week. The best time to test the A/C is May, before the shop and everyone else is busy with it. Running the A/C or defrost for a few minutes every couple of weeks in winter keeps the compressor seals oiled.
2 and 1/2 Mechanics is at 1345 Marchand St in Cyrville, Ottawa, ON, a short walk from Cyrville O-Train station. Heating and A/C testing is $120 an hour, before tax, and you approve any repair before it starts. We are open Saturdays from 8am to 3pm. Book a check online, and if the problem comes and goes, our car diagnostics page explains how intermittent faults are tracked down.
Questions
Why is my heater blowing cold air in winter?
The most common causes are low coolant, a thermostat stuck open, a blocked heater core or a blend door that no longer moves. A temperature gauge that stays low on the highway points at the thermostat. Heat on one side of the car but not the other usually points at a blend door actuator.
Why does my car fan only work on the highest setting?
On many cars the lower fan speeds run through a blower resistor, and the high setting bypasses it. When the resistor fails, only high works. It is a common and fairly inexpensive repair, though a burnt connector at the resistor should be replaced at the same time.
Why does my car A/C blow warm air?
Usually the system is low on refrigerant because of a leak, but it can also be a compressor clutch that will not engage, a failed pressure switch, a fuse or relay, or a cooling fan that is not running. Topping up refrigerant without finding the leak usually means it is warm again within weeks or months. Testing finds which it is.
Does my car A/C need to be recharged every year?
No. A car A/C system is sealed and should hold its charge for years. If it needs refrigerant, it has a leak, and the leak is what needs fixing.
Why does my windshield fog up with a greasy film?
A greasy film that comes back after wiping, along with a sweet smell inside the car, usually means the heater core is leaking coolant into the cabin. A damp carpet on the passenger side is another sign. Have it checked soon, since breathing coolant vapour is not good for you and the coolant level will keep dropping.
