The heater in my EF Fairmont works fine but sometimes if I drive the car hard in cold weather (usually around 8-12 degrees) at night the heater seems to stop working and I get cold air... It doesn't seem to matter f the temperature gauge is on L or M.... It did this tonight, I tried the Climate Control reset but that didn't help.

Then after I started driving normally again it started working fine with the temp gauge on L

First check then vacuum hose is tight on the heater tap, then check the heater tap is opening all the way...I would say you need a new heater tap about $25.

Yea I would check the tap first. Easy enough to do and it's just fixed my cold heater.

EDIT: I love the aussy prices, cheapest I could get a heater tap for here was $65