Lennox furnace alert codes

Lennox communicating systems report numbered alert codes through the thermostat rather than a flashing light, and rate each one by how urgently it needs attention.

Lennox communicating furnaces, paired with an S30, S40 or comparable thermostat, report faults as numbered alert codes shown on the thermostat itself. Lennox assigns each a priority — Information, Service Soon, or Service Urgent — which is genuinely useful, because it distinguishes a furnace that is running at reduced capacity from one that should be shut off. Non-communicating Lennox furnaces use a different diagnostic method and these codes do not apply to them.

Worth knowing before you count

Alert 229 is classed Information only and Lennox states plainly that no action is required — the furnace lit on high fire because the low fire pressure switch did not close in time, which is adaptive behaviour rather than a fault. Alert 250, by contrast, has shutting off the gas as its first documented step.

Lennox communicating alert codes

Alert code 200 — Rollout limit switch open
Alert code 204 — Check gas valve
Alert code 205 — Gas valve relay contact closed
Alert code 206 — Gas valve second stage relay fault
Alert code 207 — Hot surface igniter sensed open
Alert code 223 — Low pressure switch open
Alert code 226 — High pressure switch stuck closed
Alert code 227 — Low pressure switch open in run mode
Alert code 228 — Inducer calibration issue
Alert code 229 — Ignition on high fire — information only
Alert code 240 — Low flame current in run mode
Alert code 250 — Flame out of sequence — still present
Alert code 251 — Primary limit switch open
Alert code 252 — Discharge air temperature high
Alert code 270 — Flame failed to ignite — Watchguard
Alert code 271 — Low pressure switch open — Watchguard
Alert code 272 — Low pressure switch open in run mode — Watchguard
Alert code 273 — Flame failed in run mode — Watchguard
Alert code 274 — Primary limit switch open more than three minutes — Watchguard
Alert code 201 — Indoor blower motor communication lost
Alert code 202 — Blower motor and unit size mismatch
Alert code 203 — Invalid or unset unit size code

Other manufacturers

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.