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Lennox · Heat pump

E120 Unresponsive device

Medium severity

What the control is telling you

A component is slow to respond to the system, usually the outdoor unit answering the thermostat; often harmless and clears by itself. S30/S40 thermostat alert code 120 (shown as E120 on the unit control display).

Applies to: Lennox communicating heat pumps incl. variable-capacity inverter units in S30/S40 smart thermostat systems

Likely causes

  • Slow outdoor unit response
  • Voltage leaking between strands in the cable bundle

Test and repair steps

  1. Land only the R wire on R to load the bundle with 24 VAC.
  2. Measure each loose wire to C in AC volts 0.03-0.3 VAC is fine, up to 0.7 VAC moderate success, up to 1.2 VAC occasional success, over 1.2 VAC must be fixed.
  3. Check all wiring connections and cycle power.
  4. Clears after the device responds to any inquiry.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.17. Summarized in our own words; check the original manual for your model.

Homeowner looking at this code? See what it means for you and when to call a technician.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.