Gas Furnace Repair - How to Change the Pressure Switch

Check flame sensor , check hoses to pressure switch not split etc

These pressure switches are safety features on ALL new furnaces, if replacing the switch didn't solve the problem, you might want to make sure there is no blockage in the exhuast pipe like a birds nest, anything that would restrict airflow, is there inline fan motor in the exhaust pipe ? perhaps it's not working also.

had similar problems with same furnace. I think the 3 blinks is high limit event. either way with the power off to the furnace remove the board from the 4 clips holding it on one at a time then turn it over and look at it you will be able to tell if the board is bad because it will appear scortched and the resin will be bubbled up where it went bad. replace board and you will be fine. went through 2 in 2 years all of a sudden they are being made by a different company from the original and the 1st replacement. now all is fine. Take a good picture of the board close up so you can reconnect all the wires properly. Friend of mine got it from a wholesaler for $55.00

Yes, inspect the air intake, exhaust pipe, filter and condensate piping for blockage or moisture. Also check for a hole in the hose that goes from the small exhaust blower to the pressure switch or a bad connection on the switch or control board. You may be able to temporalily jump the switch for diagnostic purposes only.

I'm not a furnace technician. But I have worked on several Coleman trailer furnaces. So keep in mind that my terminology might not be exactly correct. But I think I can help you.

I don't know what model you are running, but most of them run pretty much the same. You have a four basic components to check. Thermostat; Ignition Control Module; Pressure Switch; and which ever of the following that you have (Glow plug, Electronic ignition, Pilot Light).

I'm assuming that you are referring to the blower as the "Rat wheel blower" (round wheel looking thing with fins on it). If my assumption is correct and if the power from the breaker box to the furnace junction box (Probably with a on and off switch on top or side), than when you give it power (from breaker box and turn thermostat up) the fan (not blower) comes on. If it doesn't there is a probably a problem with your Thermostat or Ignition Module. If it does come on and your glow plug or you igniter tries to work. Than yes, you are correct, you have to have gas pressure to let the pressure switch know that there is adequate gas supply pressure (gas- 5 to 7 W.C, propane 11 to 13 W.C). The reason for this is if the Pressure Switch doesn't feel the correct pressure. It thinks you have a leak and won't release gas for ignition. After you turn the gas on. If the furnace doesn't ignite. Make sure the Thermostat is definitely wired properly. Now we are down to one of three problems. If the Glow Plug, Electronic Ignition, or pilot light are working properly. Than we are down one of two problems. The Ignition Module or Pressure Switch. Check pressure on the Pressure Switch. If yuou don't smell gas it is probably OK. But it is best if you use a manometer. I have rarely found Pressure Switches bad. Normally if I get to that step. It is the Ignition Module. The Ignition Module kind of like the brain of the furnace. It is located behind one of your front panels. It will have approximately 10 wires going to it. Some will be going to the Thermostat, some to the Glow plug (Igniter), some to the Pressure Switch, and some going to the blower.

I have probably said more than you need to know. If I did I apologize. But to wrap it up. You were probably correct. You definitely need gas for the blower to kick on. The furnace has to light and heat up before the blower will Start. Not to keep rattling, but to check to see if the blower motor will run. Is to switch the Thermostat from "auto" to "run".

Good Luck