My Day & Night 517E electric furnace runs a long time and does not heat as well as it used to. This is relatively recent. The air from the registers _seems_ cooler and the furnace does not reach it set point most of the time. During the day, it will sometimes get to the set point, but at night, when it is cooler, it never does. The furnace has three coils and I suspect that one or more are not working. Can this be a sequencer problem? Or a coil problem? Can I replace a bad coil myself? Can I replace the sequencer myself? What should I expect to pay for these parts?
I have a some diagrams and instructions for the unit but they are not giving me a clear picture of the heater coils.

You guessed it

Heater Coil or sequencer.

If you are not comfortable with electric call a pro. If you are comfortable with electric then you can read the diagram and fix it.

Parts prices varies as does labor.
You will need to check locally for price.