I have a 2002 4 cylinder that the temp gauge isn't working, the heater is only providing lukewarm air, and my gas mileage is down by 3 miles per gallon.
any ideas? Thermostat?

 

Heater core, Thermostat, not enough coolant, coolant leak

 

Yes, thermostat first, it is stuck open, and be prepared, 2002 2.3l used a Heated thermostat on lower Rad hose, they are pricey.

You can use a 2004 2.3l thermostat, no heater and much cheaper, but you need to leave the old one plugged in so computer thinks it still there, many do this and just zap strap the old one out of the way with wires connected

Ford realized after 3 years that the heater was not needed, so only the 2001/02/03 models had this type 2004 to 2012 used the non-heated versions, and so can you
2003 could have either type, it was the transition year

All these are housings with the thermostat built in, so thermostat is not a separate part like other vehicles/engines might have


There is a Coolant temp SENDER on the engine, it will have a Red/white stripe wire and a Black/white stripe wire attached, this is for the dash board temp gauge, NOT the computer
Find it and make sure it is plugged in, if so then replace it to get temp gauge working again

 

 

Thank you, very helpful, getting right after it!