Furnace Pressure Switch Errors
A pressure switch fault locks out the furnace before the gas valve ever opens — and nine times out of ten the switch is telling the truth. Here is how to read the draft, prove the switch, and find the real blockage instead of jumpering past a safety.
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In This Guide
What the Pressure Switch Actually Does
On a high-efficiency (90%+ AFUE) condensing furnace, the draft inducer runs before the burners light. The pressure switch is a safety that proves the inducer is establishing enough negative draft to pull combustion products through the heat exchanger and out the vent. If it cannot confirm draft, the control board never energizes the igniter or opens the gas valve. No proof, no fire — that is the whole point.
Physically it is a diaphragm switch: a flexible diaphragm with a set of contacts, held open by a light spring. A small rubber hose ties the low-pressure side of the diaphragm to a sensing port on the inducer housing or collector box. When the inducer spins up, it pulls a vacuum on that port, the diaphragm moves, and the contacts close. That closed signal on the 24 VAC circuit tells the board draft is proven.
The one-sentence version
The switch does not measure "is the furnace safe" directly — it measures whether the inducer is pulling the negative pressure the manufacturer says it must. Everything downstream keys off that.
Sequence of Operation
Knowing where the switch sits in the ignition sequence is half the diagnosis. A standard single-stage condensing furnace runs like this:
- Thermostat calls for heat (W energized).
- Board runs a self-check: the pressure switch must read open right now. If it is already closed with the inducer off, the board reports "stuck closed" and halts.
- Draft inducer energizes and spins up.
- Board waits for the switch to close (draft proven), typically within a few seconds.
- Hot surface igniter warms up (or spark begins).
- Gas valve opens; burners light.
- Flame sensor confirms flame; after a delay, the blower comes on.
The switch is checked at two moments — open before the inducer runs, then closed after it runs. A fault at either moment produces a different error code, and that distinction tells you where to look.
Stuck Open vs. Stuck Closed
Almost every board on the market separates these two failures in its flash-code chart. Read the code first; it points you at completely different root causes.
Pressure switch stuck OPEN
Inducer is running but the switch never closes. The furnace usually purges, then locks out.
Most common causes: no or weak draft.
Pressure switch stuck CLOSED
Contacts read closed before the inducer runs. The board refuses to start for safety.
Most common causes: welded contacts or a shorted hose/wire.
For a stuck-open code, walk the draft path — the switch is almost always right and something is choking the inducer's ability to pull a vacuum:
- Blocked condensate drain or plugged collector box. Water backs up and blocks the draft path. This is the number-one field cause on condensing furnaces.
- Blocked flue or intake. Bird nests, ice at the termination, sagging condensate in a low spot of the PVC, or a screen matted with debris.
- Failed or slow inducer motor. Bad bearings or a weak run capacitor lower RPM and draft.
- Cracked, kinked, or water-filled sensing hose; plugged sensing port on the inducer or switch nipple.
- Genuinely bad switch — but confirm draft is present before you condemn it.
For a stuck-closed code with the inducer off, ohm the switch: if it reads continuity at rest, the diaphragm contacts have welded and the switch is bad. Also check for a hose still holding a vacuum or a pinched wire shorting the two terminals together.
Reading Switch Ratings
Every switch is stamped with a setpoint in inches of water column (iwc) and a normal state — almost always N.O. (normally open). A stamp like SPST-NO -0.60" WC means the contacts close once the inducer pulls 0.60 iwc of negative pressure and open again when draft drops below that point. Match the exact rating on replacement — a switch that trips at the wrong pressure is a nuisance-fault generator or, worse, defeats the safety.
| Furnace type | Typical switch rating | Notes |
|---|---|---|
| Single-stage condensing | -0.40 to -0.70 iwc | One negative-pressure switch |
| Two-stage condensing | -0.40 (low) / -1.00+ (high) | Dual switch or dual-pressure switch |
| 80% mid-efficiency (induced draft) | -0.40 to -0.60 iwc | Proves inducer draft, vents to Cat I flue |
| Modulating | Multiple setpoints | Switch(es) matched to each firing rate |
Ranges are typical field values; always use the setpoint printed on the switch and the wiring diagram for the specific furnace.
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Field Diagnostic Procedure
Work it in order. The goal is to separate a real draft problem from a bad switch, so you do not throw a $30 switch at a plugged drain trap.
Step 1 — Read the code. Pull the LED flash code and note whether it says stuck open or stuck closed. Watch one full call for heat.
Step 2 — Prove the inducer. Confirm the inducer energizes and comes up to full speed. Listen for bad bearings; feel for weak airflow at the flue. A slow inducer will not make draft.
Step 3 — Clear the water path. Pull and flush the condensate trap and collector box drain. Standing water here blocks draft and is the most common stuck-open cause.
Step 4 — Inspect venting. Check intake and exhaust PVC for blockage, sags holding condensate, ice at the termination, and matted screens.
Step 5 — Inspect the hose and ports. Remove the sensing hose; look for cracks, kinks, or trapped water. Blow through the switch and inducer ports to confirm they are open.
Step 6 — Measure the draft. Tee a manometer into the sensing port with the inducer running and read the actual negative pressure. Compare to the stamped setpoint.
Step 7 — Test the switch. Ohm it open at rest and closed with the inducer running, or read the 24 VAC drop across the terminals (see the example below).
Voltage-drop test across the switch
With the inducer running, put your meter across the two switch terminals on AC volts:
- ~24 VAC across the switch = contacts still open (draft not proven — switch is open or draft is too low).
- ~0 VAC across the switch = contacts closed and passing the signal. The switch is doing its job.
Worked Example: Intermittent Lockout
The call
A 96% two-stage furnace throws a "pressure switch open" flash code intermittently, mostly on cold mornings. The switch on the low-stage circuit is stamped -0.40 iwc.
Step 1: Code confirms stuck open, so the switch is not seeing enough draft. Walk the draft path.
Step 2: Inducer runs and sounds strong. Tee in the manometer at the sensing port on low fire.
Measured draft = -0.31 iwc (setpoint -0.40 iwc)
Step 3: Draft is short of the setpoint, so the switch is correctly staying open. Something is restricting the path. Inspect the exhaust PVC.
Step 4: Find a belly in the horizontal exhaust holding roughly a cup of condensate. On cold mornings it freezes and chokes the draft — hence the intermittent, temperature-driven fault.
Step 5: Re-pitch the exhaust to drain back per the install manual (typically 1/4 in. per foot). Re-measure.
Draft after correction = -0.52 iwc > -0.40 iwc setpoint
Result: switch closes, furnace fires
The switch was never the problem — it was doing exactly its job. The fix was the vent pitch, not a new part. Had the tech jumpered the switch, the furnace would have run under-drafted and risked spilling combustion products.
Safety: Never Jumper a Switch
Jumpering a pressure switch to "get the furnace running" defeats the one device that confirms combustion products are being vented instead of spilling into the space. Under-drafted operation can push carbon monoxide into the home and overheat a compromised heat exchanger. It is never an acceptable repair, temporary or otherwise.
Hard rules
- Never bypass or jumper a pressure switch to run the equipment.
- Replace with the exact rating and normal state stamped on the original.
- If you find spillage or a suspect heat exchanger, red-tag the unit and test for CO before leaving.
- Confirm adequate draft with a manometer before ever condemning the switch itself.
A pressure switch that keeps tripping is a symptom. Find what is choking the draft — water, ice, a plugged port, a tired inducer — and the switch stops complaining on its own.
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