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.