Lennox furnace Alert code 226: High pressure switch stuck closed
Alert 226 means the high pressure switch will not open. Lennox rates it Service Urgent and specifically directs checking for moisture in the pressure switch.
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 high pressure switch must open when the inducer is not producing its rated draft, and this alert means it will not. A switch stuck closed proves a draft that may not exist, which is why Lennox rates it Service Urgent rather than Service Soon. The guidance names a cause that is easy to overlook: moisture in the pressure switch. On a condensing furnace, water is produced by design, and water reaching a pressure switch will hold its contacts closed as effectively as a mechanical fault. Lennox again points to Service and Application Note H-13-07 for detail.
Most common causes
- Moisture in the pressure switch, which Lennox lists explicitly.
- Pressure switch contacts mechanically stuck closed.
- A vent blockage affecting the pressure the switch sees.
- A combustion air inducer not operating correctly, or restricted.
- Water in the sensing hoses from a condensate drainage problem.
- A switch whose operating pressure has drifted out of specification.
How to diagnose it
- Check the condensate drain for backing up, since moisture in the switch is Lennox's named cause and drainage is where it comes from.
- Check the outside vent termination for obstruction.
- Note whether the furnace is level, since drainage depends on it.
- Note whether the fault appears after long runs or in particular weather, which fits a moisture-related cause.
How to fix it
- Clear a blocked condensate drain, which addresses the source of the moisture Lennox names.
- Clear the vent termination if obstructed.
- The pressure switch, its hoses, and the inducer are technician work.
- Never jumper a pressure switch. It proves safe venting, and defeating it can allow the furnace to fire into a blocked flue.
Notes
Moisture as a cause of a stuck pressure switch is specific to condensing furnaces and is genuinely counterintuitive — the water is a normal product of the furnace working correctly, and it only becomes a fault when drainage fails.
Related
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
The two-stage pair: 226 against 225
This alert only exists because the furnace has two pressure switches — a low one proving minimum draft and a high one proving the stronger draft second stage needs — and the high switch can fail in both directions. Alert 225 is the high switch refusing to close: the furnace runs but never reaches full output, a performance complaint. Alert 226 is the opposite and the more serious one: a switch asserting a high draft that may not exist, which is a safety-proof failure rather than a comfort failure, and the reason this direction gets the Service Urgent rating while a won't-close fault reads as the furnace merely limping. The moisture cause Lennox names cuts both ways here — water in the switch or its sensing hoses can hold contacts closed today and freeze them open in a cold snap, so an alert history that alternates between 226 and 225 is itself evidence for the water explanation.
What the fix costs
Per the repair cost guide, pressure switch work runs $150–400 — and the guide's note for that line is to check for a blocked flue first, which for this alert generalizes to: find where the water came from first. A switch replaced while the condensate drain still backs up gets wet again, and the second invoice looks just like the first. The cheap outcomes are a cleared drain or an unblocked vent termination; the switch itself is the mid-range outcome; an inducer problem behind it is the $300–900 case. If water damage is found, ask the technician to confirm the furnace is sitting level — a unit tipped away from its drain quietly reproduces this fault forever.
DIY or call a pro
Yours: the water hunt. Look for standing water or mineral tracks under and around the furnace, feel whether the condensate line actually drains, and check the outside vent termination — those checks are safe with the furnace on or off and they target the exact cause Lennox names for this alert. Also worth a minute: note whether the fault follows long runs or wet weather, which is the fingerprint of moisture rather than mechanics. Not yours: the switch, its hoses, and anything requiring the burner compartment open — and the absolute line is jumpering. A jumpered high pressure switch on a two-stage furnace lets second stage fire without proof the flue can carry it, which converts a Service Urgent alert into a carbon monoxide pathway.
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.