The temp sensor behind the front plate only goes to the LCD temp display in the dash. The outside temp sensor for the ACC is along the firewall near the wiper motor. There is a cold engine cut-off temp sensor on the head but this delays high blower speed, not heat. It's so you don't get a full rush of cold air first thing in the morning.

Please be more specific about "the heater will not work." No air, no hot air, no hot air from the expected registers?

Sixto
95 S420
87 300SDL