Lennox furnace Alert code 223: Low pressure switch open
Alert 223 means the furnace low pressure switch is open. Lennox gives a detailed check list including one item that appears in no other manufacturer's guidance: check the levelness of the unit.
Applies to: Lennox communicating gas furnaces on systems using the S30, S40 or comparable communicating thermostats, as documented in the Lennox Residential Communicating Systems Alert Code Guide.
What it means
The low pressure switch proves the combustion air inducer is moving enough air before gas is released. Lennox's troubleshooting for this alert is unusually thorough, and two entries stand out. It directs checking the cold end heater box, condensate drain, and drain strainers — a condensing-furnace-specific drainage path that can back up and disturb the pressure signal. And it says to check the levelness of the unit, which matters because a furnace installed out of level does not drain condensate properly, and standing condensate is what reaches the pressure switch hoses. Lennox also points to its own Service and Application Note H-13-07 on condensing furnace pressure switch troubleshooting.
Most common causes
- A blocked vent, which Lennox lists among the inspection points.
- A combustion air inducer not operating correctly, or restricted.
- A blocked cold end heater box, condensate drain, or drain strainers at the outlet of the cold end heater box.
- Cracked hoses in the pressure switch circuit, which Lennox names explicitly.
- The unit not level, so condensate does not drain as designed.
- A pressure switch not closing at its rated pressure during a heat call.
How to diagnose it
- Look at the outside vent termination first, since vent blockage is among Lennox's inspection points and needs no tools.
- Check the condensate drain for blockage or slow drainage.
- Note whether the furnace has been moved, or whether anything under it has settled, since Lennox lists levelness as a check.
- Note the weather, since freezing conditions at the termination produce this fault.
- Note that Lennox references Service and Application Note H-13-07 for additional detail — worth mentioning to a technician.
How to fix it
- Clear the vent termination if blocked, and clear a blocked condensate drain. Both are within homeowner reach.
- The pressure switch, its hoses, the inducer, and the cold end heater box are technician work.
- If the unit is not level, that needs correcting properly rather than shimmed casually — it affects condensate drainage throughout the furnace.
- Never bypass a pressure switch. It is what prevents the furnace firing into a flue that cannot carry the products away.
Notes
The levelness check is worth remembering because it is easy to dismiss. A condensing furnace relies on gravity to move condensate to its drain, and a unit that has shifted or was never level produces intermittent pressure switch faults that look like component failures.
Related
- Alert code 226 — High pressure switch stuck closed
- Alert code 271 — Low pressure switch open — Watchguard
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
What the fix costs
Pressure switch work sits at $150–400 in the repair cost guide, with the guide's standing instruction to check for a blocked flue before the switch takes the blame. On this alert Lennox's own checklist stacks the odds toward the cheap outcomes: the cold end heater box, condensate drain and drain strainers it names are drainage maintenance, not parts, and a furnace sitting out of level is a shim-and-verify fix. The costly branches are a genuinely failed switch (mid-range) or the inducer behind it at $300–900. A useful question for the quote: which item on Lennox's list did the diagnosis actually eliminate? A switch condemned without the drainage path being inspected is the classic way this $150 fault becomes a $400 one.
DIY or call a pro
Split it the way Lennox's checklist naturally splits: the outside terminations (snow, ice, leaves, nests) and the condensate drain's flow are homeowner-inspectable with no panel opened, and levelness is a two-dollar bubble level on the cabinet top — three checks that cover a surprising share of 223s on condensing furnaces. The switch, its hoses, the cold end heater box internals and any pressure measurement in inches of water column belong to the technician, both because they need a manometer and because this switch is the interlock proving the flue can carry combustion products out of the house. The non-negotiable: no jumpers, ever, including “just to test it.” A jumpered low pressure switch removes the only thing standing between a blocked flue and a running burner.
The same fault on other brands
A low pressure switch that will not close is the most widely coded fault in furnace diagnostics — the pressure switch hub counts 47 related codes across 12 control families. The same condition Lennox calls 223 appears as 3 red flashes on the shared Goodman/York board, 9 flashes on Allied's Armstrong/AirEase/Concord/Ducane control, and 2 blinks on Rheem's *RC boards, which even publish the same style of timing detail (closure required within 10 seconds). What is genuinely Lennox-specific here is the drainage-focused checklist and the H-13-07 service note behind it.
What this looks like
From the outside, this usually shows up as the furnace starts and stops in short bursts — that walkthrough covers what to check in order, and when it stops being a homeowner job.