My gas hot air furnace was working fine, then I noticed it had started blowing cold air. When the heat cycle starts now the venter fan and the blower both start immediately, which I know is not right. The igniter never gets hot and gas never flows. The diagnostic led is blinking 4 times which the manual says means an open high limit switch. I checked the limit switch with a multi meter and it appears to be fine. Just to be sure I jumped the switch and it still said the switch was open. The model number of the furnace is PBKM-LD12N080f and appears to be manufactured by Unitary Products Group of York, Pa. Thanks for any advice.

First did you track down the switch you tested on the diagram on the back of the blower door and make sure you are testing the high limit switch and not some other switch?

Usually York is pretty accurate on there codes and that switch when open will cause the both blowers to run as a safety measure

Yup, it's the limit switch, I even checked the part number on it to make sure.

Then You are going to have to start checking devices

Find the auxilery limit part number 025-30321-700 and test it.

I checked the 2 secondary limit switches and the 3 rollout switches. None are open. I also checked the pressure switch and it seems to be operating as it should. I checked the wires on the limit switches and rollout switches for continuity and they seem to be OK. Not much more to check as far as I can see. Is there any way to test the circuit board itself? Thanks for any more advice.

Have you checked if the ignitor gets voltage or if the ignitor is defective?

No way I know of to check the board.

Unplug the igniter and see if you are getting voltage. If everything else checks out you need a new control module. Sometime if you dismount them you can see or smell what's been cooked.

I finally got back to checking the furnace. I unplugged the igniter and checked with a meter and it does not appear to be getting any voltage. I get about 80 ohms when I check the igniter itself which I believe is within the acceptable range. I also put the meter on the transformer and I get 120 volts on the primary side, but I'm not getting anything on the secondary side, which I believe should be 24 volts dc.

24 volts ac, not dc.

Quote:

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Originally Posted by MarkwithaK

24 volts ac, not dc.

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Yes, hopefully that was a typo. If your multimeter is set to DC I think it will show no voltage. Make sure you're measuring A/C.

Sounds like a transformer at this point. Check it again on the A/C setting instead of D/C on your meter just to be sure.

That was my mistake. I checked it with the meter set on AC and I got 24 volts.

If no voltage at the circuit board for the ignitor then the board might be defective.

You have already checked all other parts?

I have checked all the limit switches, rollout switches, and the pressure switch along with all the wiring associated with those switches. It all checks out OK. The igniter tests OK, but there is no voltage at the connecter that the igniter plugs into. Once I checked it correctly the transformer also checked OK. The led still flashes 4 times which is supposed to mean the limit switch is open. I'm afraid the circuit board is bad at this point. I'm not sure there is much else to check.

At this point I have to agree with the circuit boad diag. I have no idea how old the unit is or how much the circuit board costs. Make sure to mark down exactly where all the wires go before removal. See if you can find a part number on the board and post it.

If everything checks out and you're still not getting voltage from the board terminals then its probably your circuit board. Unfortunately, they are too complicated to diagnose without this kind of process of elimination.

Take some pics before you yank out the old board, or as suggested write a map. Sometimes when you get the old one out you can see or smell where it has failed.

Let us know how you make out.

I got my new circuit board today and installed it. The furnace still doesn't work, but the new board is giving me a different error. The led on the new board is flashing red rapidly. According to the installation instructions for the new board this means "Twinning error, incorrect 24v phasing. Check twinning wiring." I'm not sure what that means. Can someone tell me what the twinning wiring is?

Make sure you have reconnected the thermostat wires correctly. There should be no thermostat wires connected to the twin location on board. The twinning is only used if you have two furnaces controlled by one thermostat.

Are you absolutely possitive that you haven't missed a limit somewhere?

Try reversing the two low voltage 24 volt power wires that are from the transformer. It will change the Phase sequence. OR

You might have the 120 volt circuit reversed feeding the furnace. Check to make sure the neutral(usually white wire) is reading as a ground wire.