Lennox furnace E112: Ground not detected — system shuts down
E112 means the control cannot detect an earth ground. Unlike the poor-ground warning, this one shuts the system down. Normal operation resumes five 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
The furnace needs a genuine earth ground for two separate reasons, and only one of them is electrical safety. The other is flame sensing: rectification returns its microamp signal through the chassis, so a furnace with no ground cannot prove flame no matter how well it lights. Lennox distinguishes two states here. E117 is a warning that the ground is poor, and the furnace keeps running. E112 is no ground detected at all, and the system shuts down. That escalation is deliberate — an ungrounded gas appliance is both a shock hazard and a furnace that cannot verify its own flame.
Most common causes
- No ground conductor connected at the furnace junction box.
- A ground wire that is present but not bonded to the cabinet.
- A corroded or painted-over bonding point.
- An ungrounded branch circuit, common in older houses with two-wire wiring.
- A ground lifted during previous service and never reconnected.
How to diagnose it
- Note the age of the house wiring. Two-wire circuits without a ground conductor are a real and common cause.
- Note whether the furnace was recently installed, moved, or serviced.
- Note whether the furnace also shows flame sensing faults — the two go together and share this cause.
- This is not a homeowner check. It involves a live cabinet and a bonding path.
How to fix it
- Provide proper earth ground to the system, which is Lennox's own instruction.
- This is electrician or HVAC technician work, and in most places it is licensed.
- The system resumes normal operation five seconds after the fault is recovered.
- Do not defeat the code or bypass it. The ground is doing two jobs, and one of them is keeping people alive.
Notes
If a furnace has a history of unexplained flame sensing problems and a stored E112 or E117, the grounding is the first thing to fix and very often the only thing.
Related
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
E112 vs E117: how Lennox splits the grounding fault
The 508565-01 table draws a sharp line between these two codes, and the exact wording matters when you are deciding how urgently to act:
| E112 | E117 | |
|---|---|---|
| Manual wording | “Ground not detected. System shuts down.” | “Poor ground detected (Warning only).” |
| Furnace behavior | Stops operating until a ground is present | Keeps running |
| Recovery | Normal operation resumes 5 seconds after the fault recovers | Warning clears 30 seconds after the fault recovers |
| What it usually means | No ground path at all — a disconnected or never-connected conductor | A ground path that exists but is marginal — corroded, loose, or undersized |
The practical difference: E117 is a furnace warning you about future flame-sensing trouble; E112 is a furnace refusing to run because it can neither verify its flame nor protect anyone from a chassis fault. If your display alternates between the two, the ground connection is intermittent — treat it as E112.
Which models show this code
E112 belongs to the seven-segment E-code list documented in Lennox installation instructions 508565-01, which covers the ML193UHEK — a Merit Series Category IV upflow/horizontal furnace, installed direct-vent or non-direct-vent. That manual also states the control is polarity sensitive, that a green ground wire is provided in the furnace's field make-up box, and — relevant if this code appeared during an outage — that both polarity and proper grounding must be checked before running the furnace on generator power (the control wants 120 volts ±10% at 60 hertz). A furnace that throws E112 only when the generator kicks in is telling you about the generator's bonding, not the house wiring. Lennox communicating systems report grounding trouble differently, as alert code 117 through the thermostat; see which Lennox code list applies.
What the fix costs
There is usually no part to buy — the fix is restoring a connection, so the realistic bill is a diagnostic or service call at $75–150, often credited against any repair, plus electrician time if the branch circuit itself is ungrounded. The expensive version of this fault is the misdiagnosed one: a marginal ground mimics a dirty flame sensor, and repeated sensor cleanings at $80–200 per visit add up to more than fixing the ground once. More codes like this across brands: grounding & polarity codes.
DIY or call a pro
Call a pro — this one is not a homeowner fix, and the reason is specific to the fault. Confirming a ground means measurements on a live line-voltage circuit inside the cabinet of a gas appliance, and correcting one can mean work on the branch circuit, which is licensed electrical work in most places. What you can usefully do yourself: note whether the code appeared after service, a move, or a switch to generator power, and whether the house has older two-wire circuits — that history shortens the visit. What you must not do is bypass the code or run the furnace from an ungrounded source to get heat back; the shutdown exists because an ungrounded furnace cannot prove its own flame.
The same fault on other brands
Other control families dedicate a code to this exact failure of the flame-rectification ground reference: Allied's 2 flashes (improper earth ground), Trane's E62 (bad grounding), and the shared Goodman/York board code 9 red flashes / 9 red (grounding problem or reversed polarity). The site's grounding hub lists all sixteen such codes across ten control families — Lennox is unusual in splitting the fault into a shutdown code and a separate warning.
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.