Heater blows cold air
Cabin heat is simply hot coolant flowing through the heater matrix with air blown across it, so a cold heater means either the coolant is not hot, it is not reaching the matrix, or the air is not being directed through it. Check the temperature gauge first: an engine that never reaches normal temperature points to the thermostat, while a hot engine with a cold heater points to low coolant, air in the system, a blocked matrix or a stuck blend flap. Both heater hoses hot to the touch confirm flow and move the search inside the dashboard.
Urgency
Safe to drive, fix soon
Possible causes, most likely first
- Low coolant level
Air collects at the highest point, which is often the heater matrix, so the heater goes cold first. Check the expansion tank cold; if it is low, top up and find out where the coolant went. - Thermostat stuck open
Engine temperature stays below normal, especially at speed in winter, and the heater is lukewarm at best. Watch the gauge or live coolant temperature; an engine that cannot exceed 70-75 degrees on a motorway has a thermostat problem. - Air lock in the cooling system
Heater cold after coolant work or a repaired leak, sometimes with gurgling behind the dashboard. Bleed the system following the manufacturer's procedure, using the bleed screws and heater on full hot with the nose of the car raised. - Blocked heater matrix
Engine hot, inlet hose to the matrix hot but the outlet hose cool. Back-flush the matrix with a hose in both directions; heavy brown sludge coming out confirms it, and a matrix that will not clear needs replacing. - Stuck or failed blend flap motor
Temperature does not change when the dial is turned, or only one side of a dual-zone system is cold. Listen for the flap motor moving when the temperature is changed and read the climate module codes; a stripped flap gear is the usual find. - Faulty heater control valve
Cars with a coolant shut-off valve: the valve stays closed so the matrix never gets flow. Feel the hoses either side of the valve with the engine hot and the heater on. - Water pump impeller eroded or slipping
Low coolant flow gives a heater that is hot at idle and cools at speed, or the opposite, with an engine that runs warmer than usual. Check for a plastic impeller that has come loose on its shaft on affected engines.
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What you can check yourself
- Check the coolant level in the expansion tank with the engine cold
- Watch the temperature gauge: does the engine reach normal operating temperature within 10-15 minutes?
- Feel both heater hoses at the bulkhead with the engine hot: both should be hot
- Turn the temperature dial from cold to hot and listen for a flap motor moving behind the dashboard