Furnace Pressure Switch Troubleshooting: Open, Stuck Closed and Dropping Out

Pressure switch codes are some of the most common furnace faults, and the switch itself is usually not the cause. Measure the draft before you replace parts.

How it works

On a call for heat, the control board first checks that the pressure switch is open (proving it isn't stuck closed), then starts the inducer. The inducer pulls a negative pressure, the switch closes, and the board continues to ignition. If the switch doesn't close, or opens during the run, the board stops and flashes a code.

Two different faults

• "Pressure switch stuck closed": the switch is closed before the inducer starts. Causes are a welded switch, a mis-wired or jumpered switch, or (rarely) an inducer that won't stop.
• "Pressure switch open" or "failed to close": the inducer runs but the switch never closes, or it drops out during the run. This is the common one.

Measure the draft

1. Tee a digital manometer into the hose to the switch, or measure at the inducer port per the manufacturer.
2. Run a call for heat and read the negative pressure (in. w.c.).
3. Compare it to the setpoint printed on the switch (for example, −0.65 in. w.c.).

If the draft is weaker than the setpoint, the switch is doing its job. Find the restriction:

• Vent or intake blocked: bird nests, ice, snow, debris, a crushed pipe or excessive vent length.
• Condensate trap or drain plugged (condensing furnaces): water backs up into the inducer or heat exchanger and kills the draft. Clean the trap and the hoses.
• Hose problems: cracked, kinked or water-filled hose, or a plugged port on the inducer or collector box.
• Weak inducer: worn bearings, a damaged wheel or low voltage.
• Wind: strong wind across vent terminations can make the switch drop out intermittently.

If the draft is stronger than the setpoint but the switch stays open, check continuity across the switch while it should be closed. An open switch means replace it with the exact OEM-specified setpoint.

Never leave a pressure switch jumpered. It's a combustion safety. Bypassing it can let the furnace fire with a blocked vent and fill the house with carbon monoxide. Jumper it only momentarily for diagnosis, if at all, with the furnace attended.

Intermittent drop-outs

If the switch drops out partway through a cycle, it's often marginal draft: a partly blocked trap, water sloshing in the hose, or a vent pitched the wrong way so condensate pools. Watch the manometer over a full cycle.

FAQ

Can I replace a pressure switch with a different setpoint?

No. Use the exact part or the setpoint the manufacturer specifies. A different setpoint changes the safety margin.

Why does my pressure switch fail only in very cold weather?

Usually ice on the intake or exhaust termination, or condensate freezing in the vent or drain.

Does a pressure switch fault mean the inducer is bad?

Not necessarily. Most of the time it's a vent, trap or hose problem. Measure the draft to find out.

Sources

• HVAC School: technical tips and diagnostics