A gas furnace refuses to start when its venting system cannot prove it will remove exhaust fumes. The furnace pressure switch performs that safety check: it senses the negative pressure created by the draft inducer motor and blocks ignition when the airflow is too weak. When the switch sticks open or fails electrically, the furnace simply will not light. The diagnostic routine is short once you understand the sequence, and the same pressure-testing discipline used for home water pressure testing applies when you trace these faults.
How the Draft Inducer and Pressure Switch Work
The draft inducer is a blower mounted near the heat exchanger. At startup it spins up and pulls combustion air through the burner, across the heat exchanger, and out the flue vent, creating a pressure that is slightly lower than atmospheric on the inlet side. That negative pressure is the signal the pressure switch watches for.
The Negative Pressure Sequence
When the thermostat calls for heat, the control board energizes the draft inducer first. The inducer builds negative pressure inside the venting path, the diaphragm inside the pressure switch moves, and the switch contacts close. Only then does the control board allow ignition, so the furnace can never fire while the flue is blocked.
What the Switch Senses
The switch is calibrated to a specific pressure range, usually a fraction of an inch of water column. The draft inducer creates a measured relationship between pressure and flow inside the venting system, and the switch confirms that the blower is actually moving enough air before the gas valve opens.
The switch is a round plastic or metal disc about the size of a hockey puck, mounted with two screws near the inducer. A silicone tube runs from the switch to a port on the inducer housing, and two wires carry the signal back to the control board. Identifying it by shape and position makes the rest of the test faster.
Types of Pressure Switches and Failure Symptoms
Most residential furnaces use a single-pole pressure switch with two terminals and a silicone diaphragm. Two-stage and modulating furnaces may use two switches or a switch with multiple set points, one for low fire and one for high fire. All of them fail in the same two ways: the switch sticks open, or the electrical contacts fail.
Common Failure Modes
A stuck-open switch is usually mechanical: the diaphragm is cracked, the fitting is blocked, or the tube is disconnected. An electrical failure leaves the contacts open even when the diaphragm moves correctly. Troubleshooting guides such as this one on furnace pressure switch problems cover the same failure modes and walk through the checks in order.
| Symptom | Likely cause | Quick check |
|---|---|---|
| Furnace will not start | Switch stuck open or no voltage to the switch | Confirm the inducer runs; test the switch terminals |
| Burner ignites, then shuts off | Weak draft, blocked flue, or failed switch | Inspect the vent and the condensate drain |
| Switch contacts never close | Cracked diaphragm or loose tube | Check the tube connection; test continuity |
| Intermittent lockout | Loose wiring or unstable draft | Tighten terminals; watch the switch through a cycle |
Rule Out the Venting First
Before condemning the switch, check the things that make its job impossible: a bird nest in the vent cap, a kinked silicone tube, or a clogged condensate drain. Each of those produces the same symptom as a dead switch, and all of them are free to fix.
A draft inducer that runs but sounds weak or rattles may not build enough pressure even with a clear vent. Check the blower wheel for debris and confirm the wheel is tight on the motor shaft, since a slipping wheel produces exactly the same lockout as a failed switch.
Tools and Preparation Before Testing
Testing the pressure switch takes a digital multimeter, a small screwdriver, and a few minutes with the furnace power off. Set the meter to continuity or resistance mode for the switch test, and to volts AC for the voltage test. Wear insulated gloves and keep the furnace door switch bypassed only while you watch the startup sequence.
Gathering the Multimeter and Safety Gear
A basic multimeter is enough: the switch either passes continuity when it closes or it does not. Other pressure-operated devices in the home are diagnosed the same way; the washing machine water level switch, for example, is tested with the same continuity logic after the machine fails to fill.
Reading the Multimeter
In continuity mode, a closed switch reads near zero ohms and an open switch reads infinite resistance. In voltage mode, you should see 24 volts AC across the switch terminals when the inducer is running and the switch is closed. Write down both readings, because the control board needs to see the switch close within a few seconds of the inducer starting.
How to Test the Pressure Switch Step by Step
The test takes about 30 minutes and does not require removing the switch from the furnace. Follow the steps in order so you can tell a mechanical problem from an electrical one.
- Turn off power to the furnace and remove the front cover
- Locate the round pressure switch near the draft inducer and disconnect its wires
- Set the multimeter to continuity and touch the probes to the switch terminals; it should read open
- Reconnect the tube, restore power, and start a heating cycle
- Watch the inducer spin up; the switch should click closed within a few seconds
- Check for 24 volts AC across the terminals; if voltage is present but the furnace still locks out, the switch is bad
Confirming the Diagnosis
If the switch reads open with the inducer running and the tube is clear, replace the switch. If the switch closes but the furnace still will not light, the problem is upstream in the control board or the ignition circuit. Pressure is a diagnostic quantity in many construction tasks, from the lateral pressure of fresh concrete that formwork must resist to the air pressure that duct testing measures, and understanding what the number means is what separates a real fix from a parts swap.
After replacing the switch, run two full heating cycles and watch the sequence from a cold start. The inducer should spin up, the switch should click, and the burner should light within the control board’s timing window. A second lockout after replacement points to the venting, not the switch.
When to Stop and Call a Professional
Gas appliances deserve caution. If the furnace has a cracked heat exchanger, the vent is damaged, or the pressure switch fails repeatedly, stop and call an HVAC technician. A pressure switch that fails often is a symptom of a venting problem, not the problem itself.
Related Pressure Systems in the Home
Pressure switches show up across the house, not just on furnaces. Boilers use them to confirm circulation, heat pumps use them to protect the compressor, and appliances use them to sequence water fills. Plumbing systems rely on pressure too: gravity and pressure-assisted toilets use measured water pressure to flush, and a pressure tank keeps well systems steady.
- Draft inducer pressure switch on gas furnaces
- Air pressure switch on heat pumps and boilers
- Water level pressure switch in washing machines
- Pressure tank switch on well water systems
Where Pressure Switches Appear Elsewhere
Each device has its own set point, but the test method is the same: confirm the mechanical condition, verify the electrical contacts, and rule out the plumbing or venting around it. A multimeter and a clear head cover most of these jobs.
Preventive Checks That Keep the Furnace Reliable
Most pressure switch failures trace back to maintenance that was skipped, not to the switch itself. Change the filter on schedule, keep the condensate drain clear, and check the vent termination once a year. The inducer motor should be free of debris, and its wheel should spin smoothly.
A Seasonal Maintenance Routine
At the start of the heating season, run the furnace with the cover off and watch the startup sequence once. Listen for the inducer, watch for the switch click, and confirm the burner lights and stays lit. The same test-first discipline works on other equipment: replacing the cord and trigger switch on a circular saw follows the same pattern of confirming the fault before you buy parts.
Keep a spare tube of silicone and a few wire nuts with the furnace tools. The tube costs a few dollars and is the most common wear item in the pressure sensing circuit, so having one on hand turns a lockout into a 10-minute fix instead of a trip to the hardware store.
A pressure switch that tests good and still causes lockouts points to venting or control problems, and a switch that tests bad is a $30 to $60 part that takes minutes to swap. Either way, the reading you take with the multimeter tells you which path to follow, and that beats replacing parts on a guess.