Lennox Furnace Pressure Switch Fault: Which Codes Point to It and What They Mean

Short answer: A Lennox pressure-switch fault means the furnace hasn’t confirmed the draft condition required for ignition. The code and test procedure depend on the furnace model, so use its own door legend or manual. Vent restrictions, condensate drainage and the inducer can all affect that signal; don’t bypass the switch or assume a new switch will solve it.

For a recurring Lennox pressure-switch fault, see our furnace repair service and HVAC service in San Ramon.

The pressure switch is one of the common fault points on Lennox gas furnaces. If yours is flashing a code and the words “pressure switch” keep coming up, the fix is usually straightforward once you know where to look.

What the pressure switch actually does

The pressure switch is a small safety device that confirms the inducer motor (the exhaust blower) is running and pulling enough draft before the furnace lights. If it doesn’t sense the right pressure, it trips, and the furnace won’t fire. That’s by design because it prevents combustion with blocked venting.

On most Lennox furnaces, a pressure switch fault shows up as a specific blink code on the control board LED. Blink code 31 (pressure switch open) is one of the more common ones you’ll see across Lennox model lines. Other pressure switch codes exist depending on the model, including codes for a switch that’s stuck closed, but the exact number varies by unit. This is why you should always check the label on the inside of your furnace door first; it lists every blink code specific to that model.

If you have an SLP-series furnace (like the SLP98V or SLP99V from the Dave Lennox Signature Collection) paired with a communicating thermostat, the thermostat shows a numbered alert instead of a blink sequence, for example 223 (low pressure switch open) or 227 (low pressure switch open in run mode). The next section lists them.

What “failed open” and “cycle lockout” mean

“Failed open” means the pressure switch never closed when the control expected it to. The inducer starts, the control waits for the switch to confirm draft, the switch stays open, and the burners don’t light. “Cycle lockout” means the control stopped trying for a while after repeated failures in one call for heat.

On a Lennox communicating system the thermostat shows these as numbered alerts. The Lennox iComfort S30 installation and setup guide (document 507536-08, Table 19, pages 66 to 71) lists the furnace pressure switch alerts:

Watchguard mode is the lockout. In its limit switch row, the same table describes Watchguard as a 60-minute soft lockout. Alert 271 clears on its own after a successful ignition, and 272 clears when a heat call ends successfully. If the cause is still there, expect it to lock out again on a later call for heat.

For the open and lockout alerts Lennox lists these technician checks: measure the operating pressure (inches of water column) during a heat call and whether the switch closes, inspect the vent for blockages and the combustion air inducer for correct operation, look for a blocked cold end heater box, condensate drain or drain strainers, check for cracked hoses, and check that the furnace is level. Lennox points to its Service and Application Note H-13-07, “Condensing Furnace Pressure Switch Troubleshooting”, for more detail.

If the inducer is running and the switch still reads open, the switch isn’t seeing the draft it expects. That’s why those checks start with the pressure reading, the vent, the drain and the hoses, not with a new switch.

Don’t confuse these with outdoor unit alerts 410 to 413 (for example 411, “OD Low Pressure Switch Strikes Lockout”: the low pressure switch opened five times during one cooling or heating demand). Those are the refrigerant pressure switches on the air conditioner or heat pump, a different part with different causes.

Why it trips: most likely causes in order

1. Blocked or kinked pressure tubing. There’s a small rubber hose running from the inducer housing to the pressure switch, and it’s prone to kinking, cracking, or filling with condensate. A plugged tube means the switch never sees pressure even if the inducer is spinning fine. Check this one first.
2. Condensate drain backing up. On high-efficiency (90%+) Lennox furnaces like the EL296V or the SLP-series, combustion produces water vapor that has to drain out. If that drain line clogs, water can back up into the pressure switch port and hold it open. You’ll often see this in spring or fall when the drain hasn’t been cleaned in a while.
3. Inducer motor failing. If the inducer is weak, slow, or intermittent, it won’t build enough suction to close the switch. The motor might run and still not produce the right pressure. You can hear it: a healthy inducer has a steady hum, a failing one rattles, hesitates, or sounds labored.
4. Pressure switch failure. The switch itself does fail, especially on older units. They’re inexpensive parts, but confirm the tubing and drain are clear before you replace it. Swapping a switch on a clogged drain just puts you back in the same spot in a week.
5. Venting issues. A blocked flue, a bird nest in the exhaust pipe, or a cracked PVC vent joint on a 90%+ unit can restrict airflow enough to trip the switch. Less common, but worth a visual check from outside.

What a tech does to diagnose it

A technician will start by reading the blink code, then check static pressure on the switch with a manometer. That tells you whether the switch is the problem or the inducer isn’t building enough pressure to trip it. They’ll also inspect the drain trap and condensate line, check the hose for cracks, and look at the inducer motor amperage.

If pressure tests normal but the switch still won’t close, the switch gets replaced. If pressure is low, they trace it to the inducer or the venting before touching the switch.

What you can check before calling

A couple of things are worth doing before you pick up the phone:

• Power cycle the furnace. Flip the disconnect switch off, wait 30 seconds, turn it back on. Watch whether the fault comes back immediately or the furnace runs for a few minutes first. That tells a tech whether it’s a hard failure or something intermittent.
• Check the condensate drain from the outside. On 90%+ efficiency Lennox units, there’s a white PVC drain line coming out the bottom or side. If you can see it’s backed up or kinked at the exit point, that’s useful information to pass along. Don’t try to disassemble the drain trap yourself.
• Look outside at the exhaust pipe. If the PVC vent termination is blocked by debris, leaves, or a bird nest, that’s a visible fix. Clear the obstruction and try a power cycle.

Don’t jump the pressure switch to bypass it. It’s a combustion safety device. Bypassing it doesn’t fix the fault, it just removes the protection while the real problem stays.

Call a pro

If the fault comes back after a power cycle, or if the simple external checks don’t reveal anything obvious, you’re into pressure testing and electrical diagnosis. That’s a job for a tech with a manometer. They’ll read the actual pressure the inducer is producing, test the switch directly, and check the motor amps. In most cases it’s clear within the first 20 minutes what the real cause is.

The repair itself, whether it’s a switch swap, a drain flush and trap cleaning, or an inducer motor, involves the combustion side of the furnace. That’s not a DIY job. Done wrong, it creates a safety hazard and often voids the manufacturer warranty on other components.

Lennox parts are widely available. If the inducer motor needs replacing, lead times vary, but we can usually source one quickly.

If you’re in the East Bay, South Bay, or Tri-Valley, give us a call. We work on Lennox systems regularly and will get you on the schedule as fast as we can. A furnace repair diagnostic visit is $99, credited toward the repair. If the furnace has power and the inducer runs but the burner never lights for a different reason, see furnace igniter glows but burner won’t light.

Key takeaways

• Blink code 31 on most Lennox units points to a pressure switch open fault, but codes vary by model, so check the door label for your specific unit.
• A cracked or kinked pressure hose and a clogged condensate drain are the most common causes. That distinction is worth knowing so you can describe the symptoms accurately when you call.
• Never jumper the pressure switch because it's a combustion safety device.
• If the hose and drain look fine externally, a tech with a manometer can confirm in one visit whether it's the switch, the inducer motor, or a venting issue.

Related questions

Need help with your system? Call (925) 999-4095 or book a visit. Repair diagnostics are $99, credited toward the repair. Installation assessments are free.

Written by Andrew Kuznetsov. Andrew is the founder and owner of Bay Area HVAC Service (ADRIUM Service Solutions). He holds a California Contractor License (CSLB #1136642), EPA Section 608 Universal certification, and is factory-trained on Daikin (Houston, 2025) and Mitsubishi. He writes from direct field experience, not marketing copy.

Further reading

• Carrier Furnace Status Codes and Greenspeed Heat Pump Fault Codes , A Carrier furnace blinks a two-digit status code on an amber LED. A Greenspeed heat pump or AC sends a code like 18-11 to the Infinity thermostat. Here are both lists from Carrier's service manuals, by model, with the checks you can do before you call.
• Choosing a controller for your Mitsubishi system , Handheld remote, MHK2, wired control or the Comfort app: what to check before adding a controller to a Mitsubishi indoor unit.
• Mitsubishi heat pump lineup: which family fits your home? , Compare FX, GX, JX, HX and WX wall units, SUZ indoor styles and P-Series, then decide between a single-zone system and a multi-zone layout.