I have a strange problem.
I have an IPC/Tempstar T9MPD075F12A3 condensing furnace, roughly 12 years old. It uses a Honeywell SV9541M "SMART VALVE".
EDIT: Just found the service manual, checking it out now: Service Manual
EDIT2: Was not much help.
The furnace is short cycling. The inducer comes on, the ignitor comes on, the flame comes on, the main blower comes on and stays on for a couple of minutes.
Then everything shuts down, blower first, then inducer for about a second, and then the startup cycle happens again. This goes on indefinitely.
There are NO trouble codes. I verified that the pressure switch is working both on and off, the limit switches are NOT tripping, the fire rod sensor/ignitor combo is OK (swapped it with the downstairs furnace that is working fine).
The interlock switch is functioning normally.
The odd part is that if I remove the bottom panel where the main blower is, and defeat the interlock the furnace runs fine without short-cycling. So it seems like the extra intake air solves the problem. There are no obstructions in the intake plenum, I even removed the filter and still get the same symptoms.
If it was a matter of not enough air causing the exchangers to overheat, wouldn't the limit switches trip? And wouldn't the valve blink a trouble code? If I manually pull the spade lug off of the limit switches the furnace shuts down and blinks a trouble code. Same with the pressure switch.
What could cause short cycling with no trouble codes, that resolves itself by opening the lower panel to let more air into the main blower?
EDIT: As a test, I've bypassed all the limit/pressure switches on the wiring diagram one at a time, and the problem still occurred every time. I thought maybe the control board mounted by the main blower might have a thermal problem that was alleviated by opening the bottom panel (then intake air flows across the board) but I put some fiberglass insulation over the board and could not get the problem to occur as long as the lower panel was open.