No heat usually starts under the hood
A car heater is a small radiator, the heater core, sitting behind the dashboard with hot engine coolant running through it. The fan blows air across it and into the cabin. So when the heat goes, the first questions are about the engine, not the dash.
Is the coolant full. A slow leak drops the level until the heater core, which sits high, is the first thing to run dry, and you get cold air at idle that warms up a little when you rev the engine. Is the engine actually getting hot. A thermostat stuck open lets coolant flow through the radiator all the time, and on a minus 20 day driving the 417 the engine never gets near its normal temperature. The gauge sitting lower than usual is the giveaway. Air trapped in the system after a coolant repair can also block the heater core until it is bled out properly. Leaks, thermostats and water pumps are covered on the engine repair page.
Heater cores and blend doors
If the engine is hot and the coolant is full but the vents stay cold, we check the two hoses going into the heater core. Both should be hot. If the inlet is hot and the outlet is cool, the core is plugged with scale or old sealer, which happens when coolant has not been changed in years. Sometimes a flush clears it. Sometimes the core has to come out, and on many cars that means removing much of the dashboard, so you get the full price before deciding.
A leaking heater core shows up as a sweet smell inside the car, a greasy film on the inside of the windshield, or a damp passenger footwell.
The other common cause sits in the air box itself. Blend doors swing between the heater core and the cold side, and between the defrost, dash and floor vents. Small electric actuators move them. When an actuator’s plastic gears strip, you hear a clicking behind the dash and the door stays wherever it stopped. On dual zone systems, heat on one side and cold on the other usually means one actuator has failed.
A fan that only works on high
If the fan works on the top setting and nothing else, the blower motor resistor has almost certainly failed. The lower speeds run through the resistor to slow the motor down. High bypasses it. Resistors fail from heat, and they usually fail because the blower motor is wearing and drawing more current than it should. So we check the motor’s current draw before replacing just the resistor, otherwise the new one can fail by next winter.
A fan that does not run at all on any speed is an electrical diagnosis, a fuse, relay, motor, control head or wiring. The auto electrical repair page covers how those faults get traced.
When the A/C blows warm
The A/C works by compressing refrigerant, cooling it in the condenser at the front of the car, and letting it expand in the evaporator behind the dash. Lose some refrigerant and the system loses its cooling. Lose enough and a pressure switch stops the compressor from engaging at all.
The condenser sits right behind the grille, which puts it in the line of fire for stones from the highway and salt spray all winter. Corroded and punctured condensers are a common leak point on Ottawa cars. Other leaks happen at the rubber seals in the fittings, hose crimps, the compressor shaft seal and the evaporator. We check pressures on both sides of the system, look for oily residue at the fittings, and use dye or an electronic leak detector to find slow leaks.
If the compressor clutch does not engage, the cause can be low refrigerant, a fuse or relay, a pressure switch, or the clutch itself with too wide an air gap. A noisy compressor or one that has seized is a bigger job, and debris from it can contaminate the rest of the system.
Refrigerant, leaks and recharges
Cars built until the last several years mostly use R134a. Most newer cars use R1234yf, which is a different refrigerant with different service equipment and a much higher price per kilogram. The label under the hood says which one your car takes and how much.
Refrigerant cannot legally be vented in Ontario. The right way is to recover what is left, fix the leak, pull the system into a vacuum to check it holds and boil off moisture, then charge the exact weight the manufacturer specifies. A top up from a can without finding the leak tends to last until the next hot week.
Heating and A/C inspection time is $120 an hour, before tax, parts extra.
Fogged windows and smells
Windows that fog up and will not clear in winter usually mean moisture inside the car. Check for a wet carpet from a blocked A/C drain, a leaking heater core, or a door seal. A clogged cabin air filter also cuts airflow to the defrost vents. The A/C runs in defrost mode on most cars to dry the air, so a system that is low on refrigerant makes defrosting slower too.
A musty smell when the fan starts is usually mould on the evaporator. A sweet smell points to coolant. A burning smell from the vents needs looking at soon. To have any of this checked, book a heating or A/C inspection and describe what the vents do on each setting. If a warning light came on at the same time, a diagnostic scan may be part of the visit.
Questions
Why is my car heater blowing cold air?
The most common causes are low coolant from a leak, a thermostat stuck open so the engine never gets hot, a heater core plugged inside, or a blend door that is stuck on the cold side. If the temperature gauge reads low on the highway, start with the thermostat. If the gauge is normal but the vents stay cold, the heater core or blend door is more likely.
Why does my car fan only work on high?
On most cars that means the blower motor resistor has failed. The lower speeds run through the resistor, while high bypasses it and runs the motor at full power. The resistor often fails because a worn blower motor is drawing too much current, so the motor is checked at the same time.
Why is my car AC not blowing cold?
A slow refrigerant leak is the most common reason, often at the condenser, a fitting seal or a hose. Other causes are a compressor clutch that does not engage, a failed pressure switch or fuse, a blocked condenser, or a blend door letting heat in. Pressure readings and a leak test show which one it is.
Can you just recharge my car's air conditioning?
A recharge without finding the leak usually means it goes warm again in a few weeks or months. The system should be checked for leaks, repaired, pulled into a vacuum and charged with the exact amount of the refrigerant your car uses. Refrigerant cannot legally be vented in Ontario.
What is the clicking noise behind my dashboard?
A repeated clicking or tapping from behind the dash, often when you start the car or change the temperature, is usually a blend door actuator with stripped plastic gears. The actuator is a small motor that moves the doors directing air between heat, cold and the different vents. Replacing it is often straightforward, though some are buried deep behind the dash.
