My heater is not making the temperature to the set temp. and it feels like cool air is blowing out of vents. For example if I set my temp on the thermostat to 68 it will get to 66 and stay on it just will not reach the desired temp but if I then set it to 75 the air will get hot but once again it will stop blowing hot when it hits about 73 and then just stay on constantly. This is starting to raise my light bill and wear out the fan that is outside. Do you know the area I should start looking at and diagnosing first.

Without a brand name and EXACT model number no help can be offered.