How do you fix Which Lennox code list applies applies?

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. Full steps and safety notes are on this page.

Which Lennox code list applies: Lennox uses two code systems, and knowing which you are reading matters

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. Where the two share a number they agree — but the E-list carries codes the alert list has no equivalent for.

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, showing codes in the form E200, E223, E250, with the last ten held in a recall menu. Checked code by code against both documents, the overlapping numbers agree: 110 to 117 are the line voltage and grounding family on both, 200 is rollout on both, 223, 227, 240, 241, 250, 270 to 275 and 290 all carry the same meaning on each list. That is worth stating plainly, because it is the opposite of what is true of Rheem, ICP and York. Lennox is consistent with itself. What differs is coverage. The seven-segment list adds a supply and grounding family the alert list does not carry — E110 to E115 for line voltage and the 24 volt supply, plus E106 for a twin communication fault and E163 for a control board failure. The alert list is the larger of the two and runs the other way: twenty of its numbers, including the whole 201 to 207 blower and gas valve group and 291 to 295, have no seven-segment equivalent at all.

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.
Where the lists share a number they agree, so a number alone is usually safe to act on.
Each list carries codes the other does not, so a number that will not resolve may simply belong to the other system.

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 a code will not resolve against one list, check the other. Neither is a superset: the alert list is longer, but the seven-segment list holds the E110 to E115 supply family it does not.
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 seven-segment display looks similar.

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

Worth contrasting with the other manufacturers on this site. Rheem, ICP and York each publish lists that genuinely disagree with one another about the same code. Lennox does not — its two lists are consistent where they overlap. The site said otherwise until 4 August 2026, on the strength of a transposed pair of code numbers in the first reading of the alert guide; that error is described on the alert 241 and alert 250 pages.

All codes for this manufacturer

Lennox furnace alert codes — every code, with what differs between models.

Reading the seven-segment board's menu

On the E-code side, the 508565-01 manual documents a push-button interface next to the display that homeowners can legitimately use for reading (not fixing). Hold the button and the display cycles a menu, changing every five seconds; release to select. Choosing E enters Error Code Recall and steps through the last ten stored codes; choosing c inside that menu offers to clear them — it requires a second confirming press, and the wise homeowner never gives it before a technician has seen the list. Choosing F shows the live flame current in microamps, the number behind the flame-signal codes. Three horizontal lines (≡) mark the exit. Between calls, a blinking decimal point means idle, and H with a digit means a heat stage is running. Knowing this much converts “the light is doing something” into an exact code before anyone is paid.

What the fix costs

Knowing which code system you are reading is itself worth money: the repair cost guide's whole premise is that the fault code, correctly read, is the homeowner's leverage on the quote — a $75–150 diagnostic goes faster and lands more precisely when the caller can say “alert 223 on the thermostat” or “E223 on the board” rather than “a blinking light.” The costly failure this page prevents is cross-list confusion in the other direction: a code number quoted from the wrong Lennox list — or worse, from another brand's table, since the site's same-code-different-meanings page shows even one manufacturer's lists can contradict — can send a phone diagnosis, and the parts on the truck, in the wrong direction before anyone arrives.

DIY or call a pro

Reading codes is the homeowner side of this page; everything the codes describe keeps its own page's rules. Practical division of labor: on a communicating system, photograph the thermostat's alert screen and its history rather than transcribing (alert text is long and the numbers matter); on a seven-segment furnace, use the recall menu above and write down all ten stored codes in order, since the sequence — a ground warning followed by flame-sensing faults, say — often is the diagnosis. Never clear either history to “reset” a misbehaving furnace before a visit, and never treat a code's disappearance as a repair; both Lennox lists are full of self-clearing codes whose underlying faults persist. Then look the specific code up on this site and follow its page — the DIY line runs in a different place for a filter-and-airflow code than for anything touching gas, flame sensing or safety interlocks.

What this looks like

From the outside, this usually shows up as the thermostat screen is blank — that walkthrough covers what to check in order, and when it stops being a homeowner job.

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.