Furnace Pressure Switch: Causes, Error Codes and Fixes
Ben Amina
Aire One East Heating & Cooling, 1910 Dundas St E #117, Whitby
A furnace pressure switch is a small safety switch that makes sure your furnace is venting properly before it lets the gas light. Every time the furnace starts, the inducer fan pulls air through the combustion chamber and out the vent pipe, and the pressure switch confirms that draft is there. If it does not sense the right pressure, the furnace will not light, which keeps combustion gases from building up inside your home. In Durham Region, the most common reason a pressure switch stops the furnace is not a bad switch at all. It is snow, ice, or water blocking the airflow the switch is watching. This guide explains how a furnace pressure switch works, the seven most common causes of a pressure switch error, what you can safely check yourself, and when to call a technician.
- The pressure switch proves the inducer fan is venting combustion gases safely before the gas valve can open.
- The classic symptom is an inducer fan that runs, but the igniter never heats and the burners never light.
- Snow or ice on the outdoor PVC pipes and a blocked condensate drain are the most common causes in winter.
- Cracked hoses, a weak inducer motor, and a failed switch also trigger pressure switch errors.
- Never bypass or jumper a pressure switch. It protects your home from carbon monoxide.
Short answer: a furnace pressure switch error means the furnace cannot confirm a safe vent draft, so it will not light. You usually hear the inducer fan run, then the furnace waits, retries, and locks out, with a blinking pressure switch code on the control board.
Clear snow and ice from the outdoor vent pipes, check the filter and thermostat, then call a technician if it still will not light. Aire One East offers 24/7 furnace repair across Durham Region. Call 905-576-7600.
- What does a furnace pressure switch do?
- Signs of a furnace pressure switch problem
- 7 causes of a furnace pressure switch error
- Open and stuck closed error codes
- What you can safely check
- Furnace pressure switch repair and replacement
- How to prevent pressure switch lockouts
- Key terms in this article
- Frequently asked questions
What does a furnace pressure switch do?
A gas furnace lights in a set order. The thermostat calls for heat, and the control board starts the inducer fan, also called the draft inducer. That fan creates a slight negative pressure, or suction, that pulls combustion gases through the heat exchanger and pushes them out the vent pipe. A rubber hose connects the inducer or the collector box to the furnace pressure switch, which has a small diaphragm inside. When the suction reaches the level set for your furnace, the diaphragm moves and closes the switch. Only then does the control board heat the igniter and open the gas valve.
If the switch does not close, the board assumes the venting is blocked or the inducer is not working, and it keeps the gas off. If the switch is already closed before the inducer even starts, the board treats that as a fault too, because it means the switch cannot be trusted. Either way the furnace stops before lighting. That is the switch doing its job. Its whole purpose is to keep carbon monoxide and other combustion gases moving out of your home instead of into it.
Signs of a furnace pressure switch problem
- The inducer fan runs but nothing lights: you hear the small fan start, but the igniter never glows and the burners never come on.
- Repeated tries, then lockout: the furnace stops the inducer, waits, starts again, and after a few attempts locks out.
- A blinking error code: the control board light flashes a pressure switch open or stuck closed code.
- Gurgling or clicking: water sloshing in the inducer or collector box, or the switch clicking on and off, often points to a drain problem.
- Problems in windy or snowy weather: the furnace shuts down during storms, then works again once the vents are clear.
If the burners light and then go out after a few seconds, the problem is more likely the flame sensor. Our furnace flame sensor guide covers that symptom. If the inducer runs, the switch closes, and the igniter still does not glow, see our furnace igniter guide.
7 causes of a furnace pressure switch error
- Snow or ice blocking the outdoor vents: high efficiency furnaces vent through two white PVC pipes on the side of the house. Drifting snow, frost buildup, or ice from the exhaust can block them and stop the draft. This is the number one winter cause in Durham Region.
- A blocked condensate drain or trap: high efficiency furnaces produce water. If the drain line, trap, or condensate pump is clogged or frozen, water backs up into the collector box and blocks the pressure signal. Our furnace leaking water guide explains the drain system.
- A cracked, kinked, or loose hose: the small rubber hose can crack with age, slip off, or fill with water, so the switch never feels the suction.
- A weak or failing inducer motor: worn bearings or a damaged fan wheel may spin but not pull enough draft.
- A blocked vent or intake inside the pipe: a bird nest, debris, or a sagging pipe that holds water can restrict flow.
- A failed pressure switch: the diaphragm can tear or the contacts can stick, so the switch stays open or stays closed.
- Wiring or control board faults: a loose terminal or a control board problem can look like a switch fault.
Open and stuck closed error codes
Most modern furnaces flash a code on the control board light that you can see through the small window on the furnace door. The exact number of flashes depends on the brand and model, so always check the label inside the door. The two pressure switch codes mean different things.
| Error | What it means | Common causes |
|---|---|---|
| Pressure switch open, or did not close | The switch never sensed enough draft after the inducer started | Blocked vents, blocked condensate drain, cracked hose, weak inducer |
| Pressure switch stuck closed | The switch was already closed before the inducer started | Failed switch with stuck contacts, wiring fault |
Write down the flash pattern before you call. It helps the technician come prepared with the right parts.
What you can safely check
These steps rule out the simple causes. None of them involve opening the furnace or touching the switch, hose, or wiring.
- Go outside and clear snow, frost, and ice from around both PVC vent pipes by hand or with a soft brush. Keep at least 30 centimetres clear around them.
- Replace a dirty furnace filter.
- Set the thermostat to HEAT a few degrees above room temperature and replace weak batteries.
- Look at the condensate drain line and pump for an obvious kink, disconnection, or water on the floor.
- Turn the furnace power switch off for 30 seconds and back on once to clear a lockout. If it locks out again, stop and call.
Furnace pressure switch repair and replacement
A technician starts by finding out why the switch is open, not by swapping the switch. They check the vents and intake, clear the condensate trap and collector box, inspect the hose and ports, and measure the draft the inducer is producing with a manometer. They also confirm the switch opens and closes at its rated pressure. If the switch itself has failed, it is replaced with the exact part for your furnace, because each furnace pressure switch is calibrated to a specific pressure for that model. After the repair, the technician runs several heating cycles to confirm reliable ignition and safe venting. Because it involves the venting and combustion system, this work in Ontario should be done by a TSSA-certified gas technician.
Our team handles furnace repairs, emergency furnace repair in Ajax, and furnace repair in Oshawa across Durham Region 24/7. If the furnace has no heat for another reason, our furnace not working guide walks through the other causes.
How to prevent pressure switch lockouts
After every snowfall, check that the outdoor vent pipes are clear, especially after a storm or when snow is cleared from the driveway into a pile near them. Keep shrubs and stored items away from the pipes. Indoors, keep the condensate drain and pump clean and make sure the drain line cannot freeze if it runs through a cold area. A yearly tune-up is the best protection. A technician cleans the condensate trap, checks the hose and inducer, and tests the furnace pressure switch before winter. Our furnace maintenance guide lists everything a visit covers, and our maintenance and protection plans include it every year, with priority service in a cold snap.
Key terms in this article
- Pressure switch: a safety switch that closes only when the inducer creates enough draft to vent combustion gases.
- Inducer fan: the small motor and fan that pulls combustion gases through the furnace and out the vent.
- Collector box: the part of a high efficiency furnace where combustion gases and condensate gather.
- Condensate trap: a loop that holds water and drains condensate from a high efficiency furnace.
- Manometer: a gauge that measures small air pressures, used to test draft.
- Lockout: a safety shutdown after several failed start attempts.
Frequently asked questions
| Your furnace pressure switch questions, answered |
|---|
| What causes a furnace pressure switch to fail? Most furnace pressure switch errors are caused by something blocking the draft the switch measures: snow or ice on the outdoor PVC vents, a clogged or frozen condensate drain, or a cracked or loose hose. A weak inducer motor, a failed switch diaphragm, or a wiring fault can also cause it. |
| Can I bypass a furnace pressure switch? No. The furnace pressure switch stops the gas from lighting when combustion gases cannot vent safely. Bypassing or jumpering it can let carbon monoxide build up in your home. If the switch keeps tripping, call a licensed gas technician to find the cause. |
| What does pressure switch open mean on a furnace? Pressure switch open means the switch did not sense enough draft after the inducer fan started, so the furnace will not light. Common causes are blocked vents, a blocked condensate drain, a cracked hose, or a weak inducer motor. |
| Can snow cause a furnace pressure switch error? Yes. On high efficiency furnaces, snow or ice covering the outdoor intake or exhaust pipes stops the draft, so the pressure switch keeps the furnace off. Clearing the pipes by hand often restores heat. If it does not, call a technician. |
| How long does it take to replace a furnace pressure switch? Replacing a furnace pressure switch is usually a quick repair done in one visit, once the technician confirms the switch itself has failed. They must install the exact pressure rating for your furnace and test several heating cycles afterward. |
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References and further reading
