After you check the thermostat and wrap up in a few extra layers, make your way down to the furnace. In nearly every furnace that I have worked on, there is a small plastic viewing window where you can see a red LED (it looks like a little light). Under normal operating conditions this LED should remain constant, showing no problem has been recognized by the system. If it’s blinking, then you need to interpret what it’s trying to tell you.
In this case we are going to assume that your LED is giving you 3 blinks at a time. Three blinks usually means one of two things: 1.) There is something obstructing the exhaust airflow or 2.) The pressure switch is malfunctioning/bad.
In my example I have a furnace that has two metal covers: the first houses the burners and the motherboard (circuit control board) and the second, which is underneath the first, does basically nothing except hold in a switch to keep the furnace on. Once you take off the second cover the furnace shuts off.
Begin by shutting off the gas to the furnace via the gas shutoff valve, which should be located within two to three feet of the furnace. (Just trace the line from the furnace backwards). Now, take off both doors and set them to the side. The most common of these two problems, in my experience, is the pressure switch so we will work on that.
Looking at the blower you should see a small rubber hose running from the side of it to a small circular device (pressure switch) with two wires running to it. Carefully remove the hose from the blower and then unplug the two wires from the pressure switch. At this point you can check to see whether or not the pressure switch is working via a multimeter, with the dial set to ohms (the upside down U). This part is a little bit of a pain so…I’m sorry.
1.) Place one lead of the multimeter (that’s the pointy thing with a red or black handle) onto the metal connection where the wire was originally.
2.) Do the same with the remaining lead and connection.
3.) Now, blow into the hose and observe the reading on the multimeter. The reading should change as soon as you stop blowing. If it does not or says OL this means there is an open circuit meaning that the switch is not working properly. You need to replace this piece. (When you blow into the hose, there should be a soft clink of metal on metal if the switch is working properly).
The last two pressure switches I have bought have been around $50 dollars apiece. If you want to make certain that the switch is bad, take the two wires that were connected to the pressure switch and connect them together or “jump” them. In order to jump these wires, I use a small piece of 14-gauge wire with both ends stripped and connect one end to each of the pressure switch’s wires. Put the bottom cover back in place. You should notice the hot surface ignitor begins glowing red in 30 seconds or less (that’s the part that lights the gas). Once you see this, remove the jumper wire- your furnace should shut off.
Now, all you need to do is to buy your pressure switch and switch it out with the existing one. (Don’t forget to turn your gas back on!)
Remember to use caution when repairing your furnace. Not only is there gas involved, but you also run the risk of electric shock if you tamper with the wiring. Be Safe! Wear gloves when dealing with electrical projects and/or turn off the power.