Pardon my ignorance, but what is a "heat sensor"?

My 98 Civic Hatch gives poor mileage until the engine warms up. If your
Thermostat is broken and does not allow the engine to work at an optimum
temperature,  the engine will return poor mileage. Also, exhaust emission
will be poor until the engine decides that the car is warm enough to go
into a more efficient burn (proper air/fuel mixture). Since there's more
pollutant
coming out of the engine (because it's running too cold), it's going to put
more
of strain on the catalytic converter which has to process all the extra
pollutant.
Catalytic converter replacement can be expensive.

Pars