Lennox furnace E117: Poor ground detected — warning only
E117 is a warning that the earth ground is poor. The furnace keeps running. It clears thirty seconds after proper grounding is provided.
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
This is the softer half of the grounding pair. E112 is no ground detected and shuts the system down; E117 is a ground that exists but is not good enough, and the furnace continues to operate. That distinction is worth understanding rather than dismissing, because a warning-only code is exactly the kind that gets cleared and forgotten. A marginal ground produces intermittent flame sensing faults — the furnace works, then does not, then works again — and those get diagnosed as a dirty flame sensor over and over. If a furnace has had its flame sensor cleaned more than once and keeps coming back, a stored E117 is the explanation.
Most common causes
- A ground connection that is loose, corroded, or made to a painted surface.
- A ground conductor that is too small or too long for the run.
- A bonding screw that was never fully tightened.
- Ageing connections in an older installation.
- A ground path through a joint that has degraded over time.
How to diagnose it
- Note whether the furnace has a history of flame sensing complaints. That is what a poor ground looks like from the outside.
- Note whether previous flame sensor cleanings have stopped working sooner each time.
- This is a measurement rather than an inspection, and it belongs to a technician.
- Do not dismiss it because the furnace is running. Warning-only does not mean harmless.
How to fix it
- Provide proper earth ground to the unit and check the ground path to the system, which is Lennox's instruction.
- The warning clears thirty seconds after the fault recovers.
- Fix this before replacing any flame sensing component. It is upstream of all of them.
Notes
Lennox giving this its own code, separate from no-ground-at-all, is more useful than it first appears. Most furnaces have no way to tell you the ground is merely mediocre.
Related
- E112 — Ground not detected — system shuts down
- E273 — Soft lockout — recycles exceeded after flame failure
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
The number a poor ground drags down
The 508565-01 manual gives this board a way to make the problem visible. Holding the diagnostic push button next to the seven-segment display cycles a menu; selecting “F” enters Flame Signal mode, and the display then reads out the flame current in microamps. That is the exact measurement a marginal ground corrupts — the rectification signal returns through the chassis, so a poor ground shows up as a depressed or unstable flame-current reading even with a clean sensor and a healthy blue flame. The same manual's instruction for the related low-flame-current code is to measure neutral-to-ground voltage to confirm a good unit ground, which is the confirming test a technician will run. The E117 action line itself reads “Provide proper grounding for unit. Check for proper earth ground to the system,” and the warning clears thirty seconds after that is done.
What the fix costs
Warning-only means you can schedule this repair — it does not mean you can skip it, and the economics say do it soon. The likely bill is one service call at $75–150 plus the time to remake a bonding connection, since there is rarely a part involved. Left alone, a marginal ground matures into intermittent flame dropouts that get treated as a dirty flame sensor at $80–200 per cleaning, each fix holding only until the ground connection misbehaves again. Two of those visits cost more than the actual repair. More codes like this across brands: grounding & polarity codes.
DIY or call a pro
The measurement side of this belongs to a technician or electrician — verifying ground quality means meter work on live line voltage inside a gas appliance cabinet, and E117 specifically describes a ground that looks connected but measures badly, which is exactly the case eyeballing cannot catch. Because the furnace keeps running, the homeowner mistake here is not attempting the fix, it is deleting the evidence: this board stores its last ten error codes in the Error Code Recall menu, and clearing them before a technician has seen a stored E117 removes the one clue that separates a grounding fault from another round of flame-sensor cleaning. Leave the history alone, book the visit, and mention any past flame-sensing complaints when you do.
The same fault on other brands
Most control families that monitor grounding at all give it a single lumped code — York's 9 red flashes covers grounding, reversed polarity, or reversed transformer leads in one bucket, and Allied's 2 flashes flags improper earth ground on Armstrong, AirEase, Concord and Ducane furnaces. Lennox grading the same electrical fault into a running warning (this code) and a shutdown (E112) is the unusual part; the full sixteen-code list across ten control families 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.