[gpeck]

My in-laws got a GMT or GMTH Goodman Power Vented Gas Furnace. I changed the “ignitor” but it will not come up because the ‘vent pressure switch’ does not close!
The red led flashes 3 times which indicates the ‘vent pressure switch’ is the cause. Would you have any suggestions how to deal with that?
Thank you, George

[hvac1000]

Take the plastic/vinyl tube off pressure switch and check for water by blowing into tube. Reassemble turn main power back on and try. If no water present the switch might have bought the farm.

[gpeck]

There was no any water and I did blow in. I can hear some kind of movement inside the switch when I blew and sucked on the inlet and the air moves in and out.
So, if the pressure switch is bad the igniter will not start to glow at all, is that correct?
Thanks, George

[hvac1000]

Yes

If you have a meter you can test the switch for open and close action.

[gpeck]

The 2 leads that go to that vacuum switch, can I short them out with a piece of wire to see if the furnace comes up?
Thanks, George:cool:

[hvac1000]

I guess you can but I always use a meter to test the on and off action.

[T-Top]

This may sound like a dumb question but did you check the vent pipe and air intake to see if you had a restriction. A small leaf from a tree can make its way all the way to the air baffle on the intake at the furnace and keep it from working. A small wasp nest in the exhaust can do the same. The reason for the pressure switch is to make sure all is venting the way it should. Good luck.

[gpeck]

Well, I did not check the air flow and exhast but will do that next time I will go there.
I did use a meter and I can see how it closes but it never shuts off completely. There’s a small pinhole at the inlet.
Also, I shorted out the two wires and the LED started flashing twice so I disconnected that. I guess I’ll just get a new vent vacuum switch and replace it. Too bad there’s nowhere description how these things work.
Thanks, George

[T-Top]

You can jump the two wires out only after the inducer motor has started and is running(for test only). The control board knows if the pressure switch is open or closed with out the motor running if closed with out the motor running nothing will happen it locks out.

[gpeck]

The pressure switch worked intermittently and so using the wire trick T-Top suggested by tricking the furnace brain that there is a working switch finally identified the faulty switch not working properly. Now, how to find the right one is the next trick. So many configurations! The one I got it says on it: MPL-9300-V-0.60-DEACT-N/O-VS, B13701-42[G], DC:20/03. The price range is from $20 to $120. I guess the original are the price ones and the chip ones do not last. Now, where can I find the one that matches the one that will work on my furnace!
Thanks for any suggestions.
George

[mullyden]

You can jumper across the wires going to the switch shortly after the furnace is turned on and the 3 blinks problem should go away and the furnace should run, but only do this to test as you are still not sure that there is not a plug in vent line. The switch is a vacuum switch and should close at 0.6 inches of water pressure. You can check it by getting a piece of clear pvc tubing and a plastic tubing "T". Affix the t onto the removed switch tube and another port onto the clear tubing. Put the clear end into a glass of water. Then pull a vacuum (suck) on the third port of the t.. Water should lift about .6 inches when the switch triggers. You should be able to hear the click. If that works OK, then due check with an ohmmeter to verify the switch closes. If both of the actions work, then the switch is OK.
You then connect the clear tubing to the nipple the switch tubing was connected to. Put the other end into a glass of water and attempt to start. The vent fan should pull a vacuum higher than the point at which the switch triggered, maybe an inch. If you get no vacuum (water lift) turn the unit off and check the nipple for pluggage and clean with a small drill bit. If you still get no vacuum and you have verified that the vent stack is not plugged, I'd call a tech to fix. In any case keep your hands away from anything spinning inside the furnace and also be careful when jumpering the switch. Good Luck.