Posted by AP2U TECH SUPPORT on 9/14/2018 to HEATING/FURNACE REPAIR
In the modern age, just like automobiles, furnaces have gotten smarter. Most furnaces built since the mid 80's have got electronic control boards in which most have incorporated a "TROUBLESHOOTING" or "ERROR" Code System built into them. In most cases this will be in the form of a "FLASHING LED LIGHT" This LED bulb can be green, red or sometimes yellow/orange depending on the brand and model. In most cases if you look on the back of the door panel there will be reference to what the flashing lights mean which will lead you to the point of failure. This does not mean that part has failed so much as that it "DID ITS JOB" in the case of a thermal and/or limit switch failure but it will at least guide you to the area in which your heater has failed and with proper testing the use of these codes will help diagnose why it failed in the first place. Most furnaces will give what is called a "FLASH CODE" which means the LED will flash a certain amount of times then pause then flash the code again.
Remember- some gas furnaces will attempt to light and run up to 3 times before they go into a "LOCKOUT MODE" then flash the code so you may have to wait until the system has locked out in some cases to see this code appear.
Below is a list of basic functions that most gas fired furnaces follow as well as a few basic things to check along the way.