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totulnou 10-31-2012 08:25 AM

Furnace would not ignite!

I have a Diplomat 80MGF3 furnace that stopped working. The induced draft would start working and after waiting awhile the blower would start working, without ignition. I get a 3 flashes code from the led, meaning pressure switch failed to close. I have tested the power to the switch in and out, and i am getting +/_ 25.6V. I have checked if there is power to the valve and i am not getting anything. Please give me some guidance!

old_squid 10-31-2012 11:27 AM

The pressure switch failing to close can be due to many things, some simple and some very dangerous.

Basically the switch monitors pressure(s) (either negative or positive or both) that the venter motor developes when it runs. If a problem causes the pressure(s) to be outside of the manufacturer's specifications the switch will not allow the burner to light. This is a MAJOR safety item on all HE furnaces and is there to protect both the people in the house and the furnace itself.

Other than suggesting that you check the end of the exhaust vent from the furnace (outside of the home) for any kind of obstruction, there is honestly nothing you can do to diagnose what the problem is without the specific testing equipment required to make the measurements needed to figure out the problem. This is not one of those "change parts until it works" kind of problems.

Marty S. 10-31-2012 06:16 PM

There's a hose on the pressure switch. Follow it to the inducer and pull it off the nipple. Check that nipple for a hard crusty plug.

totulnou 10-31-2012 09:31 PM

I am not an expert but i believe that the pressure switch works. I have tested the switch with a voltmeter with and without the tube and when the tube is on it shows about 25v. with no tube i do not get any reading. There must be something else wrong. Igniter, valve, or control panel. If you have any suggestions, that would help!

JJboy 10-31-2012 09:35 PM

Should be with tube on and fan running 0V
without tube and fan running +/-24Vac

totulnou 10-31-2012 10:10 PM

Correct! I am assuming that the switch works properly since i am getting a reading of 25.5 Vac......

how 11-01-2012 12:04 AM

One way to electrically test that PS is to clip one meter lead to furnace ground and use the other lead to check for 24V going in and out of the PS. Before the furnace starts, both sides of the PS should read "0 V". As soon as the ID starts, only one side of the PS should show 24V. As soon as the ID is up to full speed, both sides of the PS should read 24V.
Your diagnostic code is saying that when the ID is up to full speed, the 24V that the board is sending to the PS is not continuing on and making its way back to the board as it should.

That code can be caused by cracked or split tubes to the PS/ tube bent enough to cut off the flow/ tube filled with condensate/ internal leak in the PS diaphram/ faulty elect switch in the PS/ damaged PS wiring or connections/ build up of "crud" just inside the ID assy where the PS tube attaches/ ID motor failing/ ID fan slipping on motor shaft/ ID fan blades dirty or broken/ degrading seal behind ID assy/ obstruction in exhaust vent/ exhaust vent that's too long or with too many elbows/ cracked exchanger/ faulty board.
If the tubes go anywhere else than between the ID and the PS, the list of possibilities gets longer.

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