Lennox furnace Alert code 117: Poor earth ground on the reporting unit
Alert 117 means a component in the system has poor earth grounding. It clears thirty seconds after the grounding is corrected, and it is worth acting on even though the system keeps running.
Applies to: Lennox communicating systems on an S30, S40 or comparable communicating thermostat. The alert names which component reported it.
What it means
A poor ground on a gas furnace is not only an electrical safety matter. Flame rectification — the mechanism that proves a flame is present — returns a few microamps through the burner and the chassis, so the ground is part of the measurement path. A furnace with a marginal ground lights normally, reads a weak flame signal, and shuts the gas off. The symptom is indistinguishable from a dirty flame sensor, which is why grounding faults get misdiagnosed repeatedly: the sensor is cleaned, the furnace works for a while as the marginal ground happens to behave, and the fault returns. This alert is the system telling you directly what would otherwise take several visits to find.
Most common causes
- A ground conductor present but not properly bonded to the cabinet.
- A corroded, loose or painted-over bonding point.
- An ungrounded branch circuit, common in older houses with two-wire wiring.
- A ground lifted during earlier service and never restored.
- A ground path through a joint that has degraded with age.
How to diagnose it
- Note whether the system has a history of flame sensing complaints or repeated sensor cleanings — that pattern and this alert together are close to conclusive.
- Note the age of the house wiring. Two-wire circuits without a ground conductor are a real cause.
- Note which component the alert names, since the ground fault is on that unit rather than system-wide.
- This is a measurement rather than an inspection and belongs to a technician.
How to fix it
- Provide proper earth ground to the component and check the ground path to the system, which is Lennox's own instruction.
- Lennox references its document Corp0123L10 for further detail on grounding.
- Fix this before replacing any flame sensing component — it is upstream of all of them.
- The alert clears automatically thirty seconds after the issue is corrected.
Notes
The seven-segment control reports the same condition as E117, and the two Lennox systems agree here as they do on every code they share.
Related
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
Why the component name in this alert matters
Unlike a flash code on a board, alert 117 arrives through the communicating thermostat with the reporting unit named — and that narrows the work before anyone opens a panel. A communicating system can have a furnace or air handler, an outdoor unit and a thermostat all on the bus, each with its own power feed, and the grounding fault is at whichever unit raised the alert, not somewhere vague in the system. A technician who knows the alert came from the furnace goes to the furnace's branch circuit and bonding point directly. Before the alert clears or scrolls out of the thermostat's history, write down exactly which component it names and when it first appeared; on an intermittent ground the alert may not be active when the technician arrives, and that note is the diagnosis.
What the fix costs
This is one of the cheapest real repairs in the code list, because the fault is a connection rather than a component: expect a diagnostic or service call at $75–150 — often credited against work done — and the labor to remake or properly bond the ground at the named unit. The cost guide lists flame sensor cleaning at $80–200 as the classic easy fix, and that is exactly the repair that gets sold repeatedly when a poor ground goes unfound; alert 117 existing at all is what saves you from paying for it twice. More codes like this across brands: grounding & polarity codes.
DIY or call a pro
Call a pro for the fix itself. Proving a ground is a resistance and voltage measurement on a live circuit feeding a gas appliance, and correcting an ungrounded branch circuit — the likely root cause in a house with older two-wire runs — is licensed electrical work. The homeowner's job on this alert is record-keeping, and it genuinely helps: which unit was named, whether the system has needed flame-sensor attention before, and whether the alert coincides with anything (a renovation, new equipment on the circuit, generator use). Do not clear the alert history to make the thermostat look tidy before the visit, and do not ignore the alert because heat still works — the system is running on a compromised flame-proving path.
The same fault on other brands
On non-communicating equipment this exact fault — a degraded ground reference under the flame-rectification signal — gets a board flash code instead of a named-component alert: Trane's E62 reads bad grounding, and the shared Goodman board code 9 red flashes lumps grounding with reversed polarity. None of those tell you which unit has the problem; this alert does. The full cross-brand list is on the grounding & polarity hub linked above.
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.