I have a 2001 Ford Taurus SE with the 3.0L Vulcan Engine

Awhile ago I started not to have heat in my car. I had the heat on but it's not nearly as hot as other cars. I've noticed when I'm just sitting in a parking lot when the cars running in park. That's when it really blows cold air, but when I drive it gets a little hotter. Its really frustrating because I'm not having any signs of the heating core going out or is already out and I just payed a mechanic 75 dollars to flush just the heater core a month ago and now it's starting to do the same thing as before. Also the temperature gauge is always fluctuating between the middle and cold. What can be wrong?

Check for a stuck-open thermostat. Drive around for about half an hour (preferably with some time on the freeway). Stop (do not shut off the engine) and open the hood. Touch the upper radiator hose (briefly, it can be hot). Upper radiator hose should be hot. If it is just warm, the thermostat is likely stuck open. Get it replaced. Flush the coolant at the same time.

A fluctuating temp gauge reading is often a sign of air pockets in the cooling system which can also lead to poor heater output… Air pocket in the heater core. Have the mechanic/shop bleed the cooling system of any air pockets and also check for cooling system leaks which can allow air into the system if levels get too low. Usually they do a pressure test or dye test to pinpoint external leaks. If the cooling system does not hold pressure but has no external leaks, then you may be looking at bad head gaskets or other internal leaks. Blown head gaskets also usually show up as white smoke coming from the exhaust while driving… Or foamy / sludgy looking oil on the dipstick or under the oil filler cap if you have other internal cooling system leaks. You might also replace the thermostat given the age of the car which can also affect cooling system performance / heater function if the t-stat stays open when it should be closed based on system temps.