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.