Lennox iComfort Error Code 31 — Pressure Switch Fault Diagnosis and Fix

Lennox iComfort error code 31 is a pressure switch fault — one of the most common furnace errors homeowners encounter. The pressure switch monitors whether the inducer motor is creating enough draft to safely vent combustion gases. When the switch doesn’t close (or opens unexpectedly), the furnace shuts down and logs code 31.

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What Does Lennox iComfort Error Code 31 Mean?

Error code 31 on a Lennox iComfort S30, E30, or thermostat-based system translates to:

Code 31 = Pressure Switch Did Not Close (or Opened During Operation)

The pressure switch is a small safety device that monitors the negative pressure (suction) created by the inducer motor. When the inducer runs, it creates a vacuum that pulls the pressure switch contacts closed, signaling to the control board that draft is established. If the switch never closes, the furnace won’t fire. If it opens mid-cycle, the furnace shuts down.

Why the Pressure Switch Fails to Close

1. Failed inducer motor — can’t spin fast enough to create draft
2. Blocked flue — condensation drain clogged, birds nest, or debris
3. Failed pressure switch — contacts worn or stuck
4. Cracked or disconnected pressure switch hose — the small rubber tube connecting the switch to the inducer housing
5. Condensate drain blocked — on 90%+ efficient furnaces, water backup can block the pressure port
6. Flue/exhaust pipe restrictions — long runs, too many elbows, or partially blocked termination cap

How to Fix Lennox iComfort Error Code 31

Step 1: Reset and Re-Test

1. Turn off the furnace at the thermostat or power switch.
2. Wait 30 seconds, then restore power.
3. Attempt a normal heat cycle.
4. If code 31 returns immediately, proceed to diagnosis.

Step 2: Inspect the Inducer Motor

The inducer (also called the draft inducer or combustion blower) is located at the top of the heat exchanger, typically on the back or side of the furnace. Listen carefully when the furnace starts a cycle:

1. When heat is called, you should hear the inducer start spinning within 30–60 seconds.
2. Listen for any grinding, rattling, or weak spinning sounds.
3. If the inducer isn’t running at all, check its wiring and test voltage at the motor terminals (should be 120V AC when commanded to run).
4. If the inducer runs but sounds weak or is slow to reach speed, the motor may be failing.

Step 3: Check the Pressure Switch Hose

This is the most commonly overlooked check:

1. Locate the pressure switch — a round disc about the size of a large coin, mounted on the inducer housing or nearby. It has a small rubber hose connecting it to the inducer.
2. Inspect the hose for:
   • Cracks or splits — even a small crack will prevent pressure buildup
   • Kinks — a kinked hose blocks pressure from reaching the switch
   • Disconnections — one end may have slipped off its fitting
3. Remove the hose and blow through it to confirm it’s clear.
4. If cracked or damaged, replace with the same diameter rubber tubing (typically 1/4” ID).

Step 4: Test the Pressure Switch Itself

1. With the inducer running (you can jumper it temporarily or wait for a call for heat), use a multimeter set to continuity or resistance.
2. Disconnect the two wires from the pressure switch.
3. Test across the two switch terminals while the inducer is running:
   • Closed circuit (continuity) = the switch is working — the problem is upstream (draft, hose, inducer)
   • Open circuit = the switch isn’t closing — either insufficient pressure or a failed switch
4. To test the switch in isolation: disconnect the hose and use a hand vacuum pump (or just your mouth) to apply suction to the switch’s barbed port. The switch should click closed when you apply enough vacuum.
5. If the switch won’t close with direct vacuum applied, replace the switch.

Step 5: Check the Condensate Drain (High-Efficiency Furnaces)

On 90%+ AFUE furnaces (two PVC exhaust pipes exiting the house), condensate water must drain freely. A blocked drain creates water backup that can:

• Fill the inducer housing and block the pressure port
• Hold the furnace offline until the water clears

1. Locate the condensate drain line — typically 3/4” PVC running from the inducer housing or collector box to a floor drain or condensate pump.
2. Pour water into the drain line inlet to test flow.
3. If water backs up, the line is blocked — clear it with a wet-dry vac applied to the drain outlet.
4. Check the condensate trap (a U-shaped section in the drain line) for debris.

Step 6: Inspect the Flue for Blockages

1. For 80% furnaces: check the metal flue pipe from the furnace to the chimney for damage, disconnections, or visible obstructions.
2. For 90%+ furnaces: check the PVC exhaust pipe from the furnace to the exterior termination cap.
3. At the exterior, verify the termination cap isn’t blocked by ice, snow, or debris.
4. Count the 90-degree elbows in your exhaust run — every 90-degree elbow counts as 5 feet of equivalent pipe length. Too many elbows can restrict draft on longer runs.

Step 7: Replace the Pressure Switch

If the switch tests bad or you’ve ruled out all other causes:

1. Note the pressure switch rating — printed on the switch body (e.g., -0.65” W.C. or -1.2” W.C.).
2. The replacement switch must match the rating exactly — using the wrong rating will either prevent the furnace from running or allow it to run unsafely.
3. Pull the Lennox model number from the furnace data plate and order the OEM switch or a compatible aftermarket.
4. Swap the switch: two wires, one hose, two screws.

Parts You May Need

Always match the pressure switch rating (in inches W.C.) exactly. Using a switch with the wrong rating is a safety issue.

When to Call a Pro

• The inducer motor needs replacement — this requires disconnecting wiring, removing the motor from the housing, and properly sealing the new motor. Not difficult, but worth professional oversight if you haven’t done it.
• Code 31 appears alongside code 32 or code 34 — multiple fault codes suggest a more serious inducer or heat exchanger problem
• You’ve checked everything and the furnace still won’t run — a licensed HVAC tech with a manometer can measure actual draft pressure and pinpoint the restriction
• You suspect a cracked heat exchanger — a partially cracked heat exchanger can disrupt combustion air flow enough to cause pressure switch faults; this is a safety issue requiring immediate professional assessment

Frequently Asked Questions

Can I jump out the pressure switch to test if that’s the problem? Technically yes — technicians sometimes do this diagnostically. But do not leave the switch bypassed. The pressure switch is a combustion safety device. Running a furnace without it functional creates a risk of flue gas (carbon monoxide) entering the home if draft fails.

How many pressure switches does a Lennox furnace have? Most 80% AFUE Lennox furnaces have one pressure switch. High-efficiency (90%+) models often have two — a draft pressure switch and a secondary pressure switch monitoring the secondary heat exchanger. Code 31 can be triggered by either one.

My Lennox iComfort shows code 31 only on cold days — why? Cold-day-only pressure switch faults usually indicate a partially blocked flue exhaust. On very cold days, condensate can freeze in or around the exterior termination cap, partially blocking the exhaust. Check the termination cap exterior when temperatures are below freezing and you’re seeing the fault.

Is Lennox error code 31 the same on all models? Code 31 is consistent across most Lennox iComfort-compatible furnace models (SLP98, EL296, ML193, etc.). However, some older Lennox models use different code numbering — check your furnace’s diagnostic label on the inside of the access panel for model-specific code definitions.

How do I find the pressure switch on my Lennox furnace? Open the furnace access panel (lower door on most models) and look for a round disc-shaped device about the size of a quarter, mounted on or near the inducer motor housing. It has two wires connected to its terminals and a small rubber hose connecting it to the inducer or collector box.

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