The gas furnace is on the blink and you need to figure out what is the reason why the heater isn't heating. Grab your electric meter, preferably a digital one, and we'll check out some of the possibilities.

In your gas furnace find out where the power enters. Check the voltage between the black and the white wires. Make sure that the black wire is the hot and the white the neutral. Reversing these wires can make intermittent problems.

We have power to the unit, so let's check further. If you turn the power on and the purge blower and the main blower both start to run then you probably have a high limit reset tripped. Look for the limit switch or switches to reset them. A little snap when you push the red button on the back will let you know that you found the right one. If there are no reset buttons then check for continuity across the terminals. If the switch is open then you need to replace it.

If your purge blower only runs but nothing else happens then you have one of two other problems. Either the ignitor is bad or the pressure switch may be bad or the flue blocked. Check the pressure switch first by seeing if you get voltage across the terminals of the switch. If the switch is closed like it should be with the vent fan running then you will not get a voltage across the terminals, this is good. Go on to check for the presence of power going to the ignitor.

Many gas furnaces have an ignitor that is very fragile and cracks easily. A small crack will keep the little bugger from getting hot and being able to light your furnace. Pull the plug apart going to the ignitor and check for power. You may have to recycle the power if you have had it on for any amount of time. Often the furnace brain will time out the ignitor try for ignition and will not have power to it continually. Also the power will only stay on for a few seconds when it does try, so you need to be ready and watch carefully.

If you are getting power to the ignitor and it is not heating up then you have a bad one. This often will only be visible by a fine line of white dust on the surface of the ignitor. This is the white line of death for the ignitor and indicates a crack in it. Purchase an ignitor that is compatible with your furnace or it may heat up too fast or not fast enough to work with your model furnace.

Hopefully this gives a few more tips to help you trouble shoot your gas furnace. As always, be very careful around and handling electrical parts. I have been zapped a few more times than I care to remember!