Furnace won’t start

Turned out to be the High Limit switch. The switch had burnt out, which caused the burner not to fire. Turns out when the limit switch senses the heat box is too hot, it shuts down the gas and forces the blower to continue so it will clear the excess heat. When the switch failed it always told the system that the furnace was over heated, so the burner would not come on and the blower would run constantly to try and clear the excess heat.

To trouble shoot this issue I used an ohm meter to test each sensor, to make sure it was open/closed as it should be. Another way to test would have been to use jumpers to bypass the sensor all together, until the faulty sensor was located.

There are a couple of things that it might be: It could be the flame sensor. I have seen them where they look clean but are dirty enough that the DC voltage being generated is not enough for the circuit board to keep the gas valve open. I would pull it out and clean it with some scotch brite or a brillow pad. be care full not to damage the porcelain base. It is generally fairly easy to remove, one or maybe two 1/4 screws holding it in place. Gently pull it out and clean it with the pad like you would if you where sanding a wooden dowel. The 2nd thing it might be (less likely) could be a roll out switch. If your air filter is dirty it would prevent enough air flow across the heat exchanger to keep it from over heating causing the roll out safety switch to open. If this is neglected for a long period of time it can cause the ROS to weaken and begin to trip at lower than designed temperature. ps. there are typically LED lights on a the circuit board that will blink error codes to help in diagnostics. It is best to capture that info and will usually point you in the right direction.