Lennox furnace E223: Pressure switch failed open

E223 means the pressure switch did not close when it should have. The inducer is running but the control never got the signal that proves the flue is clear, so it will not light the burners.

Applies to: Lennox gas furnaces with a seven-segment diagnostic display on the control board, such as the ML193UHEK, per installation instructions 508565-01.

What it means

The pressure switch is a diaphragm switch connected by a small hose to the inducer housing. Suction from a running inducer pulls it closed, and that closure is the proof the control needs before it will energise the igniter and open the gas valve. A switch that stays open means either the suction is not there or the switch cannot respond to it. Lennox's troubleshooting is unusually concrete: check the pressure in inches of water column at which the switch closes on a heat call, measure the actual operating pressure, and inspect the vent and combustion air inducer for correct operation and restriction. That turns a parts-swap into a measurement — the switch either sees the pressure it needs or it does not, and the number says which.

Most common causes

A blocked vent or intake termination outside — snow, ice, a nest, or a cap left in place.
Water in the pressure switch hose, the collector box, or a blocked condensate drain.
A cracked, kinked or disconnected hose.
An inducer running but not developing enough suction, from worn bearings or a dirty wheel.
A failed pressure switch, or one of the wrong specification fitted previously.

How to diagnose it

Check the vent and intake terminations outside. This is homeowner work and it resolves a large share of these in cold weather.
Note whether the inducer sounds normal, weak or noisy.
Note any standing water near the furnace, which points at the condensate path.
Everything inside the cabinet is technician work.

How to fix it

Clear the vent or intake termination if it is blocked.
Otherwise this is a technician job, and Lennox's own steps are measurements rather than replacements.
Never jumper the pressure switch to force the furnace to fire. That interlock is what stops it burning gas when the flue is not clear.

Notes

E223 and E224 are the two halves of the same switch. Failed open means it never closed; failed closed means it was already closed before the inducer started, so it could not prove anything.

All codes for this manufacturer

Lennox furnace alert codes — every code, with what differs between models.

This code is not a homeowner repair. Faults in this group involve the gas valve, the burner compartment, or the heat exchanger — the parts whose failure can put combustion products, including carbon monoxide, into the air the house breathes. Do not reset it repeatedly to keep the heat on. Have it looked at by a licensed technician, and make sure there is a working carbon monoxide alarm on every level of the home, including the level the furnace is on.

Sources

Lennox ML193UHEK Installation Instructions 508565-01, May 2026 — Integrated Diagnostic Codes / Status of Equipment

This site is a reference, not a substitute for a licensed HVAC technician. Gas appliances can leak carbon monoxide and cause fire. Anything involving gas lines, the heat exchanger, or the flame sensing circuit should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.