Lennox Furnace Code E111: Reversed Polarity Fixes & Costs

Last updated Sep 21, 2026 · By fixme.vip Editorial

Lennox furnace code E111: what it means

Lennox Code E111 means the control board has detected reversed line voltage polarity — the “hot” (line) and “neutral” conductors feeding the furnace are swapped. Lennox boards reference the incoming neutral to verify flame and to run their safety logic, so when the board sees line voltage where it expects neutral, it refuses to operate and posts E111.

This is a wiring integrity fault, not a component failure. Nothing inside the furnace is necessarily broken — the problem is upstream in how the 120V circuit is connected. Because it involves live line-voltage wiring, correcting it is professional work. It is also a real shock hazard: with the conductors swapped, parts of the furnace stay energized even when the unit’s switch appears to be off.

On most Lennox boards E111 holds until the polarity is corrected and power is cycled. Simply resetting power without fixing the wiring will bring the code right back. Exact wording and auto-recovery behavior vary by board revision, so confirm against the diagnostic legend printed inside your furnace’s control-panel door.

Lennox furnace code E111: common causes, ranked by probability

First, note how your furnace is powered — it changes where the fault usually is:

• Plugged into a receptacle (common on many installations): the miswire is most often at the receptacle itself or in the box behind it.
• Hard-wired to the branch circuit: the swap is more often at the furnace’s line-voltage terminal block, a junction box, a service switch, or the panel.

Ranked causes:

1. Miswired receptacle or junction feeding the furnace — hot and neutral landed backwards at the outlet or in a junction box, most often after a recent electrical repair or DIY outlet swap. This is the leading cause on plug-in installations.
2. Reversed conductors at the furnace’s line-voltage terminal block or field wiring — hot and neutral swapped where the branch circuit lands inside the furnace cabinet. More typical of hard-wired installations.
3. Reversed wiring at the breaker panel or a subpanel — neutral and hot transposed on the branch circuit serving the furnace.
4. Generator, transfer switch, or inverter with reversed or floating neutral — backup-power equipment that doesn’t maintain proper hot/neutral reference.
5. Open or lost neutral — a broken or disconnected neutral upstream can make the board read the circuit as reversed.
6. Failed or misreading control board (least likely) — the polarity-sensing circuit itself faults and reports E111 even with correct wiring.

How to fix Lennox furnace code E111: safe checks first

These are the only steps a homeowner should perform:

• Thermostat: Confirm it’s set to Heat and the setpoint is above room temperature; replace the batteries if it’s battery-powered.
• Breaker/switch: Check the furnace breaker and the wall switch (looks like a light switch near the furnace). If a breaker is tripped, reset it once. Note whether E111 returns.
• Recent work: If E111 appeared right after an outlet swap, panel job, or generator install, tell your technician — that’s your most likely culprit.
• Routine items: A dirty filter, closed registers or a clogged condensate line don’t cause E111, but it’s fine to check them while you wait.

Do not open the furnace cabinet, test wiring, or touch the terminal block. With polarity reversed, internal parts can be live even with the furnace switch off. Line-voltage polarity work is not a DIY task.

How a technician will diagnose it

A qualified technician or electrician will:

1. Verify the fault against the board’s diagnostic legend to confirm E111 is polarity, not a neighboring code.
2. Confirm how the unit is fed — cord-and-plug into a receptacle, or hard-wired — to decide where to start testing.
3. Test the receptacle or line feed with a plug tester or multimeter, checking hot-to-neutral and hot-to-ground voltages to identify a swap or open neutral.
4. Check the furnace terminal block to confirm line and neutral are landed on the correct terminals.
5. Trace upstream to the junction box, outlet, service switch, subpanel, or main panel to find where hot and neutral were reversed.
6. Inspect backup-power equipment (generator/transfer switch) if present for a floating or reversed neutral.
7. Correct the wiring, cycle power, and confirm the furnace clears E111 and completes a normal heat cycle.

If your wiring tests correct and the code persists, the board’s polarity-sensing circuit is suspect and the board is replaced last.

Symptom, cause and what to do

Lennox furnace code E111 repair cost

Costs vary by where the fault is and whether an electrician is needed. Typical US ranges:

• Service diagnostic / trip fee: $90–$180 (unavoidable on any service call)
• Correcting a miswired receptacle at the furnace: often no add-on if found during the diagnostic, up to ~$150
• Rewiring/re-landing at a junction or terminal block: $120–$300
• Electrician correcting a reversed or open neutral in the branch circuit or panel: $150–$450
• Generator / transfer-switch neutral correction: $150–$400+ depending on the equipment
• Control board replacement (only if the board misreads a correct circuit): $400–$800 installed

Because the fix is usually a wiring correction rather than a part swap, most E111 calls land near the lower end of that range once the miswire is found.

Related codes

• Lennox Furnace Code E110 — Lennox Furnace Code E110: Causes, Fixes & Costs (a line-voltage power interruption — a power loss or brownout event, not reversed polarity; low line voltage is reported under a separate code).
• Lennox Furnace Code 200 — Lennox Furnace Code 200: Causes, Fixes & Costs (a separate code in the Lennox alert set; check the diagnostic legend for your specific control board for its documented meaning).