Lennox E112 Error Code

What does Lennox E112 mean?

E112 indicates that the Lennox communicating control cannot detect a proper earth ground connection. The control monitors the ground integrity of the equipment; when ground is absent or inadequate, it shuts down the system as a safety measure. The alert clears five seconds after a proper ground is established and the system detects it.

Symptoms

• E112 or 'No Ground Connection' displayed on the thermostat.
• System is shut down by the control and will not operate.
• Alert appears after installation on an ungrounded circuit or after electrical work.
• May coincide with E117 (Poor Ground) if grounding is marginal rather than absent.

Common causes

• Ground wire not connected at the equipment terminal block.
• Missing or broken ground wire in the branch circuit wiring.
• Ground wire connected to neutral instead of the equipment grounding conductor.
• Loose ground connection at the panel, junction box, or equipment.

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Diagnostic steps

1. Turn off power and inspect the ground terminal at the equipment

   De-energize the circuit and locate the ground terminal (typically green screw or marked GND) on the furnace or air handler terminal block. Confirm a bare copper or green-insulated ground wire is securely connected.

2. Verify continuity of the ground path to the panel

   With power off, use an ohmmeter to measure resistance between the equipment ground terminal and the panel grounding bus. Resistance should be less than 1 ohm. Higher resistance indicates a broken or high-resistance ground conductor.

3. Check ground connections at the disconnect and junction boxes

   Inspect every junction or disconnect box between the panel and the equipment. Ground wires must be continuous and connected at each box. A missing pigtail or loose wire nut can break the ground path.

4. Restore power and verify E112 clears

   After confirming the ground path is intact, restore power. The system resumes normal operation five seconds after detecting a proper ground. Confirm E112 is cleared.

When to call a professional

If the ground wire is absent in the wall wiring or at the panel, a licensed electrician must run a new equipment grounding conductor. Running a ground wire through an existing circuit is governed by NEC 250.130(C) and requires professional assessment. Do not operate the equipment with E112 active — the absence of ground is both a shock hazard and a fire risk.