Basic Furnace or Heating System Troubleshooting

If you are not getting any response from your furnace and your furnace is not working, then the first thing you want to check is that there is power coming in.

If you have 120VAC coming in, then check to see if you have 24VAC coming from the transformer. If you do not have the 24VAC from the low voltage transformer then the transformer has burnt out. Power surges and especially brown outs (low voltage) can cause the transformer to burn out.

If there is 24VAC coming from the transformer secondary then check to see if there is a fuse in the low voltage circuit. Sometimes the fuse on the circuit board and often are an automotive type fuse that plugs into the circuit board. Sometimes the fuse will be an inline fuse that will be in a black round tube type holder. If the fuse is blown do not replace with a higher amperage fuse, you will cause more damage, or blow the transformer.

If the fuse is blown then unhook the t-stat wires and check again. This will tell you if the short that blows the fuse is external to the furnace or internal.

Now if all of that checks out then the next thing would be to check the low voltage safety circuit. Things such as door switches and high limit switches are all possibly included in this circuit, if any of them are open the heating system will not work.

As a last resort, the problem could be in the circuit board. They do go bad and can shut everything down also. Most circuit boards will also flash a LED light to give you a code for what is wrong and why the furnace is not working.
Count the number of flashes and then check the inside of the front cover to see what the code represents.

This will give you a basic progression of things that you can check through. Using an electrical meter is essential to being able to diagnose and troubleshoot the heating system. If you do not have one and/or do not know how to use an electric meter proficiently, then find someone to help you that does know how to use one well to help you.