540,000 kms Canadian climate. It happened once last year at -30 C. Recently, it was very cold again and the car ran ok but no heat and the gauge was going up to hot - it never gets that high, even in hot summers. One thing - I used the block heater on both occasions and I never used other than those times. Without using the block heater it does blow warm air (-20C currently). I did note today that the coolant was a bit low perhaps from overheating the other day? Would my block heater affect the thermostat? No coolant leaks noted under the car.
The block heater would not affect the thermostat, typically a 35 degree temp is the point my block heater cuts off. First step in my book would be to check the coolant for temperature rating.
Good idea on the coolant temp rating - I will check it tomorrow and post back.
There might be air trapped in the cooling system. Try this.
Get engine up to operting temperature. With the engine idling, loosen the upper radiator hose clamp and with a small flat bladed screwdriver or something simular, insert in between the upper radiator hose and the radiator hose neck. Allow the engine to continue to idle until all the air is purged out of the cooling system, or until heat comes out of the vent system.
Tester
My first thought is icy slush forming in the cooling system. Why it would happen only when you use the blockheater? No clue. Unless you only use the block heater when it is extra cold maybe.
My first thought was slushing (icing) in the system. Which compels me to ask how old your coolant is, how it’s mixed (ratio?), and if you’ve had other cooling system issues???