Furnace Pressure Switch Stuck Open: What It Means

Start with the water, because it usually is

The condensate drain and trap. On a high efficiency furnace, the one with two white plastic pipes, the exhaust condenses into water that has to drain away. When the trap or the hose blocks, water backs up into the collector box, the draught cannot develop properly, and the switch opens. Goodman says its pressure switches are there to catch a blocked condensate drain as well as a blocked flue.

Take the trap off, tip it out, rinse it, and put it back. Take a photo first so it goes back the same way round: the trap and the drain, in detail.

The hose. A thin rubber tube from the inducer housing to the switch. Pull it off at both ends and look through it. Look for a crack, a kink, an end popped off or a plug of water, and replace a bad length of hose.

The flue, the intake and the draught fan

The pipes outside. Go and look at them. On a high efficiency furnace there are two, usually white plastic, coming out of a wall. Snow drifted over them after the first storm of winter is a typical cause, and so are leaves, a wasp nest, a bird nest and a screen iced over. On an older furnace with a metal flue into a chimney, the same applies to the chimney.

The draught fan itself. Listen at the start of a cycle. It should spin up cleanly and sound steady. One that groans, hesitates, or sounds like it is struggling is not making the suction the switch wants, and the wheel inside them packs with dust exactly like a blower wheel does.

The inducer housing. A crack in the plastic housing, or a gasket that has gone hard, leaks away the suction before it reaches the hose.

Only now, the switch. They do fail, and the diaphragm inside can perish with age. It is a cheap part. It is simply not the first thing.

The code, and what the timing says

Count the flashes on the board through the little window and read them against the label inside your own furnace door, never a list from another brand.

It fails before the draught fan runs at all. That is not a switch stuck open. Goodman says a switch stuck closed stops the draught fan starting, and it has its own code. Otherwise go back to the beginning: the five free checks.

The draught fan runs, then everything stops. That is the picture on this page.

It runs and lights, then quits a few seconds later. Different fault, usually the flame sensor: what happens after ignition.

It works fine in mild weather and fails on cold mornings. Ice. Either the pipes outside or condensate freezing in a drain line that runs somewhere cold.