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 241, by contrast, has shutting off the gas as its first documented step.

How Lennox furnaces signal codes

A Lennox code reaches you one of two ways, and where you read it tells you which table applies. On communicating systems the code appears as a numbered alert with text on the thermostat screen — no flash counting involved. On furnaces like the ML193UHEK the code appears as an E-number on a seven-segment LED display on the control board itself, with the last ten codes held in a recall menu. Unusually among the manufacturers on this site, the two lists agree wherever they share a number, so a bare Lennox code number is normally safe to act on. What differs is coverage: the seven-segment list carries a line-voltage and grounding family (E110 to E115, plus E106 and E163) the alert list does not, while the alert list carries twenty numbers — including the whole 201 to 207 blower and gas valve group — with no seven-segment equivalent. One trap: a decimal point in the code, as in E2.1, means it is not a Lennox code at all. That form belongs to Trane, whose display looks similar.

The Lennox codes people look up most

The Watchguard family dominates: 270 and E270 mean the burner failed to ignite through all its retries, the single most common path to a cold house. The pressure switch pair 223/E223 (switch will not close) and its Watchguard escalation 271 point at draft and venting. 250/E250 is the open primary limit — an airflow question first. 240/E240, low flame current in run mode, is the code a dirty flame sensor produces. And on communicating systems, 201 and 292 cover a blower motor that has stopped talking or will not start, which is its own repair category on these variable-speed systems.

Which Lennox furnaces these pages cover

The alert codes are documented from the Lennox Residential Communicating Systems Alert Code Guide (document 100017), which applies to furnaces paired with an S30, S40 or comparable communicating thermostat. The E-codes come from the ML193UHEK installation instructions and apply to furnaces carrying that seven-segment control board display. Older non-communicating Lennox furnaces without a seven-segment display diagnose differently and are not covered by either table — for those, the code legend inside the access panel is the reference.

What the common Lennox fixes cost

Alert 240's low flame current usually resolves with flame sensor cleaning at $80–200. An open igniter behind 207 or E290 runs $150–350 installed. Pressure switch codes 223 and 271 are $150–400, with a blocked flue ruled out first; a limit trip behind 250 is $150–350 plus the airflow diagnosis. Inducer codes 228 and 294 land at $300–900, where brand parts availability drives the spread. Blower codes 201, 292 and 295 involve the communicating variable-speed motor — $900–1,500+ installed at that tier — and E163 is genuine control board territory at $400–900.

Lennox communicating alert codes

Alert code 116 — High 24 volt secondary voltage
Alert code 117 — Poor earth ground on the reporting unit
Alert code 200 — Rollout limit switch open
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
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 224 — Low pressure switch stuck closed
Alert code 225 — High pressure switch will not close
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 241 — Flame out of sequence — still present
Alert code 250 — 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 275 — Flame out of sequence — Watchguard
Alert code 276 — Furnace calibration failure, or high pressure switch fault
Alert code 290 — Ignition circuit fault — Watchguard
Alert code 291 — Heating airflow below minimum — Watchguard
Alert code 292 — Indoor blower motor will not start — Watchguard
Alert code 294 — Inducer motor overcurrent — Watchguard
Alert code 295 — Indoor blower motor overheating
Alert code 344 — Y1 relay stuck on the furnace control
Alert code 349 — O to R jumper cut on the furnace control
Alert code 370 — Interlock switch open — no 24 volts at the DS terminal

Lennox seven-segment diagnostic codes

E106 — Twin communication fault
E110 — Low line voltage
E111 — Line voltage polarity reversed
E112 — Ground not detected — system shuts down
E113 — High line voltage
E114 — Line voltage frequency out of range
E115 — Low 24 volt supply
E117 — Poor ground detected — warning only
E163 — Furnace control board failure
E200 — Hard lockout — rollout circuit open
E204 — Gas valve mis-wired
E223 — Pressure switch failed open
E224 — Pressure switch failed closed
E227 — Pressure switch opened during ignition or run
E240 — Low flame current in run mode
E241 — Flame sensed out of sequence — flame still present
E250 — Limit switch circuit open
E270 — Soft lockout — retries exceeded, no flame current sensed
E271 — Soft lockout — retries exceeded, pressure switch opened
E272 — Soft lockout — recycles exceeded, pressure switch opened
E273 — Soft lockout — recycles exceeded after flame failure
E274 — Soft lockout — recycles exceeded, limit circuit opened
E275 — Soft lockout — flame sensed out of sequence, signal now gone
E290 — Ignitor circuit fault — failed ignitor or triggering circuit
Which Lennox code list applies — Lennox uses two code systems, and knowing which you are reading matters

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.