Extreme cold caused thermostat problem

Last week -30F degree temp caused no heat inside the car. Later I took it to the mechanic and he said thermostat is out. and it is $500 job.
I changed that sucker some years back, it is under the intake manifold and difficult to take out.

I am thinking to change or flush the antifreeze, anyone had any luck with that?

If the thermostat is stuck open, flushing etc won't do any good.

As aleekat said if it is a thermostat problem the only fix is to replace the thermostat. With the thermostat being stuck open the coolant can never reach operating temperature. Fresh coolant won't fix that.

If the 'stat didn't close at -30, it's probably never going to close. A flush to clear it out would be nearly miraculous.

BTW - It was a few degrees cooler here, and my two daily drivers stayed very low on their temperature gauges, but still well above 160°F.