Hi,
I have a Bryant plus 80 furnace. The serial number shows it was built in 1994. I have been getting the 34 code (ignition proving failure) over and over. I have had this trouble in the past. I have found that if I let the furnace go through its lighting cycle (blower, inducer, ignitor) that I can get the gas to feed if I push the panel the control board is mounted on as the gas switch clicks. This took many hours of trial and error, but I could get it to light, and if I let off the pressure on the mounting bracket gently it would stay lit. I put the covers back on gently and that would last for weeks.

Time passed, and the problem returned, and I could only occasionally get my method to work. I resigned myself to needing another control board, thinking that somehow pushing the bracket was doing something to help the board light the burners. The blower was vibrating a lot (which I solved for now by shimming it with a two by four under it until it gets in the right position) and I thought that vibration may have damaged the board. I ordered another used board online and installed it today.

The furnace won't light. I checked all the wiring, and checked the grounds to see if anything had been compromised. All seemed fine. I touched the socket driver on the bracket and noticed that sound of the slight electrical hum would change. If I apply more pressure, the buzz changes sound, and there is a spot where it almost disappears. I tried to hold the bracket this time and slowly let it out after the first click (heating the ignitor) and the second (opening gas valve). Sure enough I could get the gas to fire when I got the bracket in just the right position, and I could repeat that over and over again. I put the covers back on, but this time it lasted about five, maybe eight, minutes and then shut off.

I thought maybe the wires on the transformer (the only other thing mounted to the bracket) were loose and that changing the angle was maybe working to deliver the voltage to the gas valve. I took the transformer off checked the wires, and replaced it. My method no longer works, I I cannot get the bracket into the right position to light.

So, I think the board is fine. I think there is enough gas pressure because the furnace does light when I can get it to, and the flame seems right. I have cleaned the flame sensor thinking that that might be the problem—it does not appear to be grounding anywhere.

I am mystified as to why my method to light the furnace has been working—there is definitely a perfect angle where the gas valve opens, but I cannot figure out why that would be so.

Any suggestions would be gratefully accepted.

Have you checked the wiring to the gas valve? Could be a loose or dirty connection. Possibly an internally broken wire.

Make sure the wire from the flame sensor has a good connection on the board.