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Discussion Starter · #1 ·
Good evening,

I have a York YP9C furnace. When I went to switch on the heat, nothing happened. Looking at the control board (FCB), i have 3 quick red blinking lights to indicate a pressure switch issue, however this is due to the inducer motor not turning on.

I have diagnosed to board myself by confirming the FCB is getting 120v in and 24v from the Transformer. I bypassed the Thermostat with connecting R to W to rule it out. Inducer motor does work when connected directly to 120v. Testing the inducer connection on the board reads only 104v which is not enough to drive the inducer. Everything I have read and viewed on you-tube seems to indicate a faulty FCB. So, I ordered a new FCB, installed it and the inducer motor did briefly come on but then started to slow down until it stopped and would just buzz. In checking the inducer voltage, it too showed a reading of 104v.

Is it possible I have 2 bad FCB’s or is something drawing the power away from the inducer motor? Has anyone experienced this?
 

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Bypass the thermostat again. When the inducer runs, wait till it stops, and then see if your actually getting 120 volts to the board.
 

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Discussion Starter · #3 ·
I monitored the voltage into the FCB and it read 116 to 118v and did not change as the inducer started to slow down. Interestingly, it start speeding up again, the furnace ignited then It started to slow down and stopped and the furnace shut down. Now the induce remains stopped with a buzzing sound. Seem if I let it sit, it will start up briefly until something starts to interfere with the power to the inducer motor.
 

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Keep in mind that this is a modulating furnace and the inducer is likely variable speed. I haven't looked into this model but the board may be changing the voltage, frequency or both to vary the inducer speed. It may be normal to not get straight 120v going to it from the board.
 

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Discussion Starter · #5 ·
I believe just the blower motor is variable speed. If I look at the wiring diagram, the inducer motor appears be straight 120v.
Text Technical drawing Floor plan Diagram Plan
 

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Speed control is probably built into the board.
Can't imagine a modulating furnace having a inducer that only runs at high speed. See if there's a service manual.
 

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Discussion Starter · #7 · (Edited)
You are right! The inducer is modulating:
FEATURES

Modulating heating operation includes:
  • Modulating gas valve, inducer and circulating blower
  • Modulating operation from 100% to 35% input in 100
increments with nearly constant temperature rise.
 

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Discussion Starter · #8 ·
I believe I have found the issue. The modulating furnace has a transducer pressure switch which when unplugged, allows the inducer motor to run normally. In looking at the transducer wires, there is a bit of corrosion from moisture that has dripped on it over the summer. I have ordered a new one which should do the trick.
 
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