If your furnace is turning on and then immediately turning off again - over and over again, it's called short-cycling. This can eventually cause your system to break down if you don't make fix the problem. It can also cause you to have a large energy bill as well. The problem with your system may be a dirty flame sensor. It's a repair you can do all on your own, but if you aren't sure about making repairs to your furnace yourself call a professional furnace repair technician. Read on for instructions to make the repair yourself if you feel up to the job.

What You'll Need For the Repair:

• Sandpaper or emery cloth
• Screwdriver
• Clean rag

Instructions:

1. Before you begin, you'll need to turn off the furnace. Turn it off with the shutoff switch on the furnace itself or, shut off the power at the breaker to your system. Do not simply turn off the furnace on your thermostat; there is still power going to the unit even if it isn't turned on with the thermostat.
2. Next, open up the control panel on the front of the furnace to find the flame sensor. The flame sensor is an L-shaped mechanism that is located near the pilot. Take a look at the flame sensor and remove it with the screwdriver. 
3. Take the emery cloth or piece of sandpaper and gently wipe away any debris that may be stuck on the sensor. When you're finished, you can use a clean rag to wipe the sensor down further to be sure all of the debris is removed.
4. After you're sure that the sensor is clean and free of debris you can replace the sensor in the furnace the same way you removed it. If there are any issues with the sensor such as damage or cracks, replace the sensor with a brand new one. You can find the replacement part at your local HVAC supply store. Make sure you purchase the right flame sensor for your particular furnace. Write down the information about your furnace model and model number.
5. Turn the power back on to your system and turn the furnace on to be sure it is working as it should.

If you are still having an issue with your furnace short-cycling or have any other problems that you aren't sure how to repair, call a furnace repairs technician for help to get your furnace working as it should.