Furnace would not ignite!
One way to electrically test that PS is to clip one meter lead to furnace ground and use the other lead to check for 24V going in and out of the PS. Before the furnace starts, both sides of the PS should read "0 V". As soon as the ID starts, only one side of the PS should show 24V. As soon as the ID is up to full speed, both sides of the PS should read 24V.
Your diagnostic code is saying that when the ID is up to full speed, the 24V that the board is sending to the PS is not continuing on and making its way back to the board as it should.
That code can be caused by cracked or split tubes to the PS/ tube bent enough to cut off the flow/ tube filled with condensate/ internal leak in the PS diaphram/ faulty elect switch in the PS/ damaged PS wiring or connections/ build up of "crud" just inside the ID assy where the PS tube attaches/ ID motor failing/ ID fan slipping on motor shaft/ ID fan blades dirty or broken/ degrading seal behind ID assy/ obstruction in exhaust vent/ exhaust vent that's too long or with too many elbows/ cracked exchanger/ faulty board.
If the tubes go anywhere else than between the ID and the PS, the list of possibilities gets longer.
Last edited by how; 11-01-2012 at 01:35 AM.