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Discussion Starter · #1 ·
Hello all, Needing a bit of help here. I've got a Ducane furnace that was installed in 2007. Lately is has started having an intermittent startup failure. When I call for heat, there is a what sounds like a relay click from somewhere inside, and NOTHING else. The heartbeat light on the control board changes from steady heartbeat to a 3 flash error. Which is a pressure switch error. It may fail like this 10-15 times, and then all of a sudden work fine the next 1, 2 or 3 cycles, and then fail again. When it does work, it works flawlessly. I have called in a technician and without any testing at all, told me the control board is failing and wants $500 to replace it. His reasoning is that with nothing happening such as the inducer fan or anything starting that the board is at fault. He spent 5 minutes of browsing at the furnace to come up with this conclusion. I'd like the input fro mall you fine folks on this to see if you all agree with his diagnosis before I let him go ahead. Any help would be appreciated.
 

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Pressure switch may be sticking closed and preventing startup.
A meter would be able to tell quite a bit more the next time it faulted.
 

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Discussion Starter · #3 ·
The way I understand it though is that the inducer fan starts up, then tests the pressure switch and if those tests pass, then the lightup sequence takes place.. correct? The problem I'm having is the inducer isn't ever starting when it's in fault so the startup sequence is never getting to the pressure switch
 

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It’s not a test, it’s just a simple circuit.
When the inducer starts it closes the pressure switch contacts. When the unit satisfies the gas valve is de-energized, the unit runs for a short post purge and the inducer shuts off. Then the pressure switch opens.
If that circuit sticks closed, the inducer likely will not start.
 

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Discussion Starter · #5 · (Edited)
I tested across the 2 leads of the pressure switch with an ohm meter and got nothing, which to me indicates the switch is open. I have also discovered over the past few hours that when the furnace fails, if I cycle the thermostat on-off-on-off-on...etc, the furnace will eventually always start up. Sometimes 6 or 7 cycles of the thermostat, but so far it will always eventually start. So whatever the problem is, it is intermittent and can be corrected with many call for heat cycles
 

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When the furnace doesn't start when you call for heat, with a multimeter check for 120 volts to the inducer motor. If there is no voltage going to the inducer, a relay on the control board is probably going bad, carefully tap on the relays on the control board if the inducer motor will not start during a call for heat. See if that causes the inducer to get power. If this is the case, then yes you need a new control board. A $500 job? Not in my book, but rates may be higher in your area.
 
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