Lennox furnace Which Lennox code list applies: Lennox uses two code systems, and code 250 means different things on each

A Lennox furnace reports faults either as alert codes through a communicating thermostat, or as E-codes on a seven-segment display on the control board. The numbers overlap, and code 250 means an open limit circuit on one and flame sensed out of sequence on the other.

Applies to: All Lennox gas furnaces. Read this before acting on a Lennox code number, because Lennox uses the same numbers in two different systems.

What it means

Lennox publishes two separate diagnostic tables. The Residential Communicating Systems Alert Code Guide covers furnaces on an S30, S40 or comparable communicating thermostat, where the code appears on the thermostat screen as an alert with text. Newer furnaces such as the ML193UHEK carry a seven-segment LED on the control board itself, which shows codes in the form E200, E223, E250, and holds the last ten in a recall menu. The two lists share most of their numbering, which is what makes them dangerous together — most codes agree, so a reader has no reason to suspect the ones that do not. Alert code 250 is flame sensed out of sequence with the flame still present, which is a gas valve concern. E250 is an open limit switch circuit, which is an airflow problem. Acting on the wrong one sends you to entirely the wrong part of the furnace.

Most common causes

The two tables document different control platforms sold under one brand.
The communicating alert codes are read from a thermostat; the E-codes are read from a display on the control board inside the furnace.
Most numbers agree between the lists, so the ones that do not are easy to miss.
Lennox does not print the letter E on the thermostat, so a code quoted as '250' does not identify which list it came from.

How to diagnose it

Establish where the number was read. A code on the thermostat screen is an alert code; a code on the small display inside the furnace cabinet is an E-code.
An E prefix, or a code recalled through a push-button menu on the control board, means the seven-segment list.
If the number came second-hand and the source is unknown, treat it as ambiguous rather than guessing.
When both readings are plausible, act on the safer interpretation first — flame sensed out of sequence is the one that involves gas.

How to fix it

Nothing to fix here. This page exists so a number is not read against the wrong table.
Once the system is identified, follow the matching page on this site.

Notes

This is the third manufacturer on this site with two contradictory code systems, after ICP and Rheem. In every case the documents are individually correct and the conflict only appears when a reader has a number and no context.

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

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.