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.