So you want to do your own furnace tune-up...

It is possible that you can do some inspection of your furnace.  I really wouldn't recommend you doing the repairs yourself - rather leave that to a trained professional - but if you want to look, here's what you should examine.

(Make sure you shut off your system before you go poking around.  Usually it's that light switch next to your furnace - or be even safer an trip the breaker).

 - Filter.  Make sure you are replacing your filter at worst quarterly and in a lot of cases, monthly.  A clogged filter reduces your airflow and can allow unhealthy air to circulate back into your house.

From here I would recommend a pro, but here is what they should do…

 - Heat Exchanger.  Inspect this for cracks and separation.  If you notice something odd get help quickly.  Left alone, cracks in your heat exchanger can allow dangerous carbon monoxide leakage into your living space.

 - Burner.  Wipe down the burner with a damp rag.  A dirty burner can result in unwanted deposits and shorten the life of your unit.

 - Flue.  Check the flue for obstructions, corrosion or leaks to ensure your fumes are exiting your home safely.

 - Thermostat.  Some of the more modern, computerized, digital thermostats run on batteries.  This might be a good time to put new ones in to keep your system humming along.  You could even modernize with a smart thermostat.

For my money you can check your system yourself if you want to, but it is money well spent to have a professional come in and give the whole system a tune-up.