Trying to fix the heating problem

There is something extremely wrong with the oil furnace in our car, plus I can’t figure out what the problem is. I mean, last week, the heating was now working just fine. I remember it because that day was the day that the weather began cooling off for the first time this season, and we turned the thermostat in the vehicle over from the a/c to the heating, plus the vehicle started heating up almost right away. I remember thinking that the two of us were lucky to have a vehicle with such an amazing type of oil furnace in it. We live in a really chilly climate plus so the two of us rarely even have to use the a/c that’s in our car. The oil furnace is another story, though! We end up having to use the heating in the vehicle on an annual basis for weeks at a time plus that’s why I suppose I’m going to have to take it in to have it worked on. No matter the problem, it cannot be that big, I don’t think. It’s not like the air vents have stopped working or anything close to that. The air vents still blow air through them but now the air is lukewarm instead of hot. That’s fantastic for now, when the temperature outside is still pretty warm, but when the temperature starts cooling down even more, having a well working oil furnace is going to be essential for us. I wish I knew more about cars in general, plus I really wish I knew more about the heating plus cooling systems in vehicles! That would be really helpful for myself and others at this point in my life!
a/c set up