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Matthew, HVAC Technician
Category: HVAC
Satisfied Customers: 2014
Experience:  14 years experience in the HVAC field of service, installation, Manual J & D calculations
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I have a N8MPN075B12A1 as well. It stopped heating the

Resolved Question:

I have a N8MPN075B12A1 as well. It stopped heating the apartment. Blower motor still works. 2 flashes saying pressure switch stuck closed. By-pass pressure sw and everything runs but the fails 4 flashes:limit sw stuck open. I have three others in the basement. Swapped blower motors, pressure sw and control boards with no fix. Any other suggestions?

Submitted: 7 years ago.
Category: HVAC
Expert:  Matthew replied 7 years ago.

So when you bypass the pressure switch the furnace does light and work fine or doesn't?

Customer: replied 7 years ago.

Furnace initiates the start sequence. "exhaust fan" starts, relays click no light off, smell of gas. No ignition. 30 seconds, 4 flashes appear indication problem with limit switch.

Expert:  Matthew replied 7 years ago.
how are you bypassing the switch exactly the order at which you jump across it matters very much. You unplug the wires then let the inducer start up then you connect the two wires together. Is this how you are bypassing it?
Customer: replied 7 years ago.
That is not how I was bypassing it. I would put the jumper in then start operation.

Under normal operation without any funny business:
Thermostat set to Heat, Auto/ON set to ON. Temp set to 90.
Furance&blower motor turned on. Blower motor begins. Green indicator shows Steady W/ flashing (normal op).

Upper heater assy does not function at all. After thirty seconds green indicator shows two flashes-pressure sw closed. No other heater functions are running. Inducer motor does not run.

When I pull the wires off the press sw and jump them and keep jumper open. Inducer motor starts to run. No other functions begin. Indication shows 4 green flashes, limit sw open.

Second time, when I pull the wires off the press sw and jump them, inducer motor starts to run. I pull the jumper wire and I get steady green w/flash = normal ops. after about 40 second inducer will power down
Expert:  Matthew replied 7 years ago.

Find the red wires that go into the plug on the control board. Cut them about 6 " away from the control board plug so that you leave yourself enough room to reconnect them with wire nuts when we are done. Take the two red wires coming out of the plug and wire nut them together so that it forms a loop going out of the plug then right back into the plug.

Now unplug the yellow and orange wire going to the pressure switch and start up the furnace. once the inducer draft motor starts up hook/jump this yellow and orange wire together and keep it together the whole time. Tell me if the hot surface igniter glows this time.

Customer: replied 7 years ago.

Just to verify, my two red wires one comes in from the thermostat and my other red wire goes out of M2, to the blower motor.
For my system, they wired the press sw with two blue wires.
Connection diagram is 329530-101 rev a

Expert:  Matthew replied 7 years ago.

Hummm, no that is not the diagram. I have for this furnace that diagram you have is for a n9 which is 90% furnace and that is a whole different ball game.

Customer: replied 7 years ago.

Weird. The placard model number n8mp075b12a1 yet the number diagram and placard on the door is 329530-101 rev a. It does give the correct connection diagram with what i am showing here. Would a couple of pictures help? If so where can I send them.

Expert:  Matthew replied 7 years ago.

I have the diagram you gave me and it shows the two blue wires connecting the pressure switch, but this diagram shows a smart valve with 3 plugs in it and the pressure switch and limits all tie into the smart valve (gas valve) is this correct?

Also what type of vent pipe comes off the furnace PVC or metal

Customer: replied 7 years ago.

Correct, a valve model #SV9541. Three electrical connections to the valve, pressure sw is wiring is blue.

Expert:  Matthew replied 7 years ago.

Ok chances are that is your problem. These smart valves are known for getting a mind of their own. The limits and the pressure switch tie into the biggest plug on the valve hold down on this plug and then start the furnace up and see if you can get it to make proper connection. These connections get weak and the valve will start doing all kinds of things. The sad part is the only fix is a new valve if it is indeed the problem.

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