Hi-
I have a '97 Protege and the heater occasionally does not work. When this happens, the temp gauge also doesn't register or move, even after driving for several minutes. And the thermostat was recently replaced. Does the engine temp trigger the heater or is that all mechanical? Is there a chance the temp gauge sending unit is bad? Appreciate any/all ideas.

 

find the heater core hoses (goes thru the the firewall ) , are they hot ? does the fan work? check fuse if no. is there enough fluid in radiator? check overflow resevoir for fluid level. (DO NOT OPEN radiator while radiator is hot.) Feel the hose from the t-stst to the radiator ? supposed to be HOT, check lower radiator hose, again supposed to be hot.
BE extra careful when doing the checks with the engine turned OFF , Too many moving parts and easy to get badly hurt by fan or belts.
Just a short check list , post more on your finding under the hood