Frigidare FG6RK gas furnace intermittently won't light


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Old 11-20-21, 12:50 AM
J
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Frigidare FG6RK gas furnace intermittently won't light

The furnace sometimes works for days or weeks, but then may stop working for days or weeks. I have the owner's manual which describes the steps to start heat. When the system fails to light, everything works right up until the gas valve should open (i.e., 24 VAC shows up on W terminal, pressure switch verifies as open, inducer motor starts, pressure switch verifies as closed, igniter heats up for 30 seconds), but rather than sending 24 VAC to open the gas valve, the igniter is deactivated, and I get a 3-flash code that supposedly means "pressure switch stuck closed." I've had the voltmeter on the pressure switch, and I see 24 VAC coming and going as expected, though.

The inducer motor will briefly stop (about 1 second), restart, then the whole sequence repeats. This will continue indefinitely. The manual says that if a no-flame condition is sensed, only 5 retries will be attempted, and then I'd get a 4-flash code, and the system would lock out for an hour before trying again. In other words, my system apparently isn't looking for flame, and therefore must know that it hasn't tried to open the gas valve. I've had the voltmeter on the valve, and can confirm that 24 VAC is not being sent.

Every once in awhile, gas will briefly turn on just before everything gets turned off for the 3-flash code failure.

Is the gas failing to turn on in time? Is that a known failure mode? (Controller board is a NORDYNE 624628)

Thanks in advance for any light you can shed.
 
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Old 11-20-21, 02:19 AM
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Welcome to the forums.

A three flash fault means "pressure switch stuck open"....... not closed.
You've got a problem in your draft inducer/flue/exhaust system.

Put your voltmeter directly across the pressure switch. You should measure 24vAC. When the inducer starts that voltage should drop to 0v and remain there for the entire time the inducer is running. By the code.... yours is not.

First place is to check where the pressure switch line connects to the inducer. There is a tiny hole in the bottom of the fitting that clogs easily. It needs to be cleaned with a tiny drill bit, small piece of wire or a pin. It must pass thru the hole to clean correctly.

 
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Old 11-22-21, 10:01 PM
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If you are getting 5v on one side and 0v on the other side means the switch is completely open.
If it was closed you would get 5v on both sides.
Some boards use 5v at startup until a call for heat.

As I mentioned.... make sure the vacuum line and fittings are clean.
If the pressure switch doesn't always get the correct pressure it will exhibit intermittent ignition.
 
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Old 11-22-21, 05:24 PM
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Thank you for the troubleshooting tips, PJmax

When power is first applied, I see 5 VAC across the pressure switch. I also checked each lead individually, and see that one is very close to 0 (0.1 VAC) and the other is at 5 VAC. Is this already indicative of a problem?

When I ask for heat, I briefly see the voltage across the pressure switch move to 24 VAC, then the inducer motor starts, and the voltage drops to 0 VAC (i.e., the pressure switch is indeed toggling from open to closed).

I see that both of these orange wires go to the controller board. Is it time for a new board? Thanks.
 
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Old 11-22-21, 07:23 PM
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I see 5 VAC across the pressure switch

>Sounds like a corroded connection upstream, I'd expect 24v or 0v unless the pressure switch is a rheostat.
 
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Old 11-23-21, 10:08 PM
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OK, I pulled the vacuum line and blew it out with some compressed air. It seemed like it was already clear. I pushed a small drill bit up into the coupler that goes into the inducer. I couldn't really tell if there was anything in there or not.

The voltmeter readings across the pressure switch remain the same as before, i.e., 5 VAC when the system is idle, 24 VAC when I turn on the heat just before the inducer starts, then 0 VAC once the inducer starts. Checking the voltage at the gas valve, the system never sends 24 VAC... it instead just turns the igniter off. I assume the board is seeing some error condition, but it seems pretty clear that the pressure switch is doing its job -- is there some sort of sensor to see that the igniter is hot prior to opening the gas valve?
 
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Old 12-06-21, 03:00 PM
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For anyone following this thread for clues on their own problem that sounds like this one, replacing the NORDYNE 624628 control board fixed this problem.

FYI, the new board also shows 5V across the pressure switch prior to the inducer motor starting (i.e., 5V is what it's supposed to be with this control board type).

One final tip is that I saved quite a bit of money by buying a used control board off of eBay from a seller named ohiomusic1. He is an HVAC tech who salvages parts from replaced, working furnaces. He tests the components that he sells, which is of course not a guarantee that it will work for you, but it felt safer than buying a board with zero history. I have no relationship with ohiomusic1 other than being a satisfied customer. YMMV.
 
 

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