Car does not heat up. Do I need thermostat?

What is your question?

It used to heat up to normal temp one day stay cool the next now doesn't get hot at all. It blows warm air but not hot even in subzero temperatures lately. Also the fans are on as soon as the car is started even if its cold. I thought maybe the thermostat was stuck open. The check engine light seemed to go off when the temperature goes up to normal which it hasn't done in awhile and it used to go on and off but it said it was gas cap