I have a York high-efficiency gas furnace. When the startup cycle begins, the inducer fan comes on, then it cycles off and on continuously. The ignitor never energizes. I have opened the input and exhaust pipes at the furnace to eliminate possibility of clogged vent lines. I checked condensate drains. The inducer motor appears to be running fine when it is on. I checked the pressure switches. They are cycling between 24 V and 0.1 V, which I assume means they are working (i.e. closing when the fan is running and opening when the fan is not running). If I unplug the rubber hose between the inducer fan and the pressure switches, the inducer fan runs continuously, but the ignitor still doesn't energize. I hear a little clackety-clack sound from the area of the fan/control board. I'm guessing that when the inducer fan comes on, and the pressure switch closes, and it is time for the ignition cycle to continue to the next thing, something is failing (maybe a relay?) causing the cycle to discontinue, which turns off the inducer fan, then it all starts over again, causing the symptom of the fan cycling off and on continuously. Any suggestions on if I'm on the right track, and what I should look for next?

 

FIrst off I am an no a pro. I think one possibility you should confirm is the furnace gas valve is open. The second is the thermostat may not be calling for heat, if it has a battery confirm it is charged.

 

That furnace should have a control board with a diagnostic LED on it. It may be visible thru a sight glass in the blower door. When the furnace fails it will blink a code which you can post here and is also listed on the inside of the blower door.

 

When the thermostat calls for heat, you should see 24 VAC switched on to the W terminal and that voltage should stay on until the thermostat shuts the furnace off.

Whenever the 24 VAC to the W terminal is on, the inducer motor should turn on and stay on as long as that voltage is present.

If that's not happening, replace the circuit board. The symptoms you describe
are characteristic of a bad circuit board.

 

I have confirmed that there is 24 V at the W terminal, and it is steady even while the inducer fan is cycling. The ignition cycle diagram in the manual indicates the induction fan should come on high briefly before the pressure switch closes, then the induction fan should switch to low. It appears that instead of switching to low, it goes off, which causes the pressure switch to open, starting the whole cycle over again. It seems that it could either be a faulty motor or a faulty control board. I should be able to check that out with a voltmeter.

 

The induction fan runs fine at high and low speed when I put 120 V to each of its input wires. The signal coming from the control board to the low speed wire of the induction fan seems erratic. It jumps up to 120V (or more) very briefly, then falls to zero. It may just be that my voltmeter can't keep up with its cycling. I guess I will replace the control board next, unless anyone has any further suggestions on less expensive things to check first. I'm guessing that the relay switch to change from high speed to low speed is not switching properly. I'm seeing it on the circuit diagram, but I can't find the actual switch. I assume it must be inside the control board unit. Anybody know if that is true, and is there anything to be done about it other than replacing the control board?

 

Have you checked the hoses between the inducer fan and the pressure switches?
Make sure both ends of the hoses are clear and the entire length of hose is clear.
Most importantly make sure the ports on the inducer where the hoses attach are clear. Those ports are especially vulnerable to clogging up with crud. Any blockage/restriction can cause erratic ignition issues. You can use a paper clip to clean the hose ends and inducer fan ports.

 

Yes, I checked all the hoses. I haven't actually checked inside the hose port of the induction fan housing. But the pressure switch does close when the fan comes on high. I guess maybe it's possible when the fan switches to low that a partially clogged port might cause the pressure switch to open again. I'll check that. Thanks for the suggestion.

 

The erratic voltage you have measured confirms that the circuit board is defective and needs to be replaced.