Lennox E117

What does Lennox E117 mean?

E117 is a warning-only alert indicating poor — but not absent — earth grounding on the furnace or air handler. Unlike E112 (no ground), E117 does not shut down the system. The alert clears automatically 30 seconds after the grounding issue is corrected. Poor grounding can contribute to communication noise (E105) and control board sensitivity to electrical interference.

Symptoms

• E117 or 'Poor Ground' displayed on the thermostat — system continues to operate.
• May appear alongside E105 (communication problem) if noise coupling is occurring.
• Alert clears and returns intermittently if the ground connection is marginal.
• No operational disruption — this is a warning, not a shutdown condition.

Common causes

• High-resistance ground connection at the equipment terminal block or panel.
• Ground wire undersized for the circuit or corroded at a connection point.
• Ground path passing through a loose junction box connection.
• Ground wire in contact with the neutral conductor upstream, creating a floating ground.

Diagnostic steps

1. 01

   Measure ground resistance at the equipment

   With power off, place an ohmmeter between the equipment ground terminal and a known-good earth ground (cold water pipe, panel ground bus). Resistance above 1 ohm indicates a poor ground path.

2. 02

   Inspect ground wire connections at all junction points

   Trace the ground wire from the equipment to the panel and inspect every junction box for loose wire nuts, corroded conductors, or missing ground pigtails. Tighten and clean all connections.

3. 03

   Check the equipment ground terminal screw

   Confirm the green ground screw or ground terminal on the equipment terminal block is tight. A loose ground screw on the equipment itself is a common cause of E117.

4. 04

   Verify line voltage and low voltage grounding per Service Application Note H-01-09

   Per Lennox documentation, refer to Corp0123L10 (or H-01-09 on newer documentation) for additional grounding information specific to communicating systems. Verify both line-voltage and 24VAC circuit grounding is correct.

When to call a professional

If the ground path is intact at the equipment but resistance remains high, the fault is in the branch circuit wiring. A licensed electrician must inspect and repair the grounding conductor. In older homes with two-wire (ungrounded) circuits, adding a ground requires a licensed electrician per NEC 250.130(C).