Changing the hot surface ignitor on a gas furnace is a repair that can easily be accomplished with minimal tools. If you have this type of ignition system I recommend keeping a spare ignitor. They are much cheaper than a service call and can prevent a night or weekend without heat.
When replacing the ignitor the first step is to turn off the electrical power to the furnace. The power switch is usually (not always) located on a junction box on the side of the furnace and has a round screw in fuse under a cover beside the switch. Next remove the front cover of the furnace and locate the burners. The hot surface ignitor will be located next to the burners on one end and protrude into the path of the flame. It has a white molex plug with 2 wires attached. Unplug the molex plug from the wiring harness. When looking at the ignitor in most applications you will see a small screw with a 1/4" hex head. You will need a 1/4" nut driver (longer and magnetized helps). Remove the screw and the ignitor should come out. Reinstall in reverse order. Do not touch the black element of the new ignitor as this will shorten the life of the part. Be careful not to bump the ignitor against the surrounding parts as the ignitor is very fragile.
If the ignitor has failed there will typically be a white line across the center of the element. If you look closely at the line you will see a small crack. Notice the white area with the visible crack in the picture below. If you do not see a crack and are familiar with a volt meter you can check voltage to the ignitor. Unplug the molex connector and power up the furnace. The draft inducer should run for 30-90 seconds before the ignitor is energized electrically. Ignitors can be powered with between 24 and 120 volts depending on your model. Some ignitors require 70-80 volts. Connect the volt meter to the molex connector from the furnace and leave it connected during the process. After starting the furnace the connector should have the proper voltage starting 30-90 seconds into the cycle depending on the design of the furnace. If you read the proper voltage the ignitor has failed. If you do not read the proper voltage during the inducer run cycle most likely the circuit board has failed.
Do not forget to turn the power back on.