Lennox furnace E111: Line voltage polarity reversed
E111 means line and neutral are swapped in the supply to the furnace. Lennox's action is to reverse the line power wiring; the system resumes normal operation five seconds after it is corrected.
Applies to: Lennox gas furnaces with a seven-segment diagnostic display on the control board, such as the ML193UHEK, per installation instructions 508565-01.
What it means
Reversed polarity is one of the classic hidden causes of a furnace that lights and then shuts down, and this control is unusual in naming it directly instead of burying it inside an ignition fault. The reason it matters is flame rectification: the control proves flame by pushing a few microamps through the burner and the chassis back to neutral and ground. That circuit only works when line and neutral are the right way round. With them swapped, the furnace can light perfectly and still fail to prove flame, so it shuts the gas off within seconds — and every part in the ignition system tests fine. On most brands this hides behind an igniter or flame sensor code. Here the furnace simply says the wiring is backwards.
Most common causes
- Line and neutral swapped at the outlet or receptacle feeding the furnace.
- Line and neutral swapped in the furnace junction box, often from a previous installation.
- A miswired branch circuit at the panel.
- A replacement disconnect switch wired incorrectly.
- An extension or repair to the circuit made without checking polarity.
How to diagnose it
- Note whether the furnace has ever worked correctly. A furnace with reversed polarity from the day it was installed usually has a history of ignition complaints.
- Note whether recent electrical work was done — a new disconnect, a moved circuit, a panel change.
- Confirming polarity requires measuring live line voltage, so it is technician or electrician work.
- If ignition parts have already been replaced without fixing the fault, this code is the likely explanation.
How to fix it
- Reverse the line power wiring, which is Lennox's own instruction.
- In most places correcting reversed polarity at an outlet or panel is licensed electrical work.
- The system resumes normal operation five seconds after the fault is corrected, with no reset needed.
- Do not have flame sensors or igniters replaced while this code is present. They are not the problem.
Notes
Several brands pair reversed polarity with an igniter fault in a single code — Goodman, Trane and York all do. This furnace separating them is a real diagnostic advantage.
Related
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
What the fix costs
There is no part in this repair at all — Lennox's action line is simply to reverse the line power wiring, and the furnace resumes on its own five seconds after it is corrected. The realistic bill is a service call at $75–150, or an electrician's equivalent if the swap is upstream at a receptacle, cord cap or panel rather than in the furnace's make-up box. The money this code saves is the point: without it, reversed polarity presents as a furnace that lights and drops out — a symptom that gets billed as flame sensor cleanings and igniter swaps until someone finally checks the supply. The 508565-01 manual is blunt that this control is polarity sensitive; a stored E111 turns a multi-visit mystery into a one-visit wiring fix.
DIY or call a pro
Correcting polarity means opening line-voltage wiring, so the fix is licensed work — but the circumstantial evidence is yours, and for E111 it is unusually telling. This fault does not develop with age; wires do not swap themselves. It appears after an event: a new installation, a receptacle or panel repair, a generator or transfer switch pressed into service. The manual's generator section says polarity must be checked before running this furnace on temporary power, so an E111 that shows up only when the generator does has found its cause without anyone opening anything. Name the most recent electrical event when you book the visit, and do not run the furnace on the suspect supply in the meantime — with line and neutral swapped, parts of the circuit that should be dead sit hot, which is a shock risk to the next person who opens the cabinet.
The same fault on other brands
Swapped line and neutral gets a dedicated code across at least six control families, all documented on the grounding & polarity hub: Allied's 1 flash, Carrier's rapid flash, Goodman's EA on the newer displays, and Trane's E61 all read reversed polarity. The unanimity is the tell: every manufacturer that monitors this considers it worth stopping for, because flame rectification fails quietly with the wires crossed.
What this looks like
From the outside, this usually shows up as the burner lights and then drops out a few seconds later — that walkthrough covers what to check in order, and when it stops being a homeowner job.