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Lennox · Heat pump

E421 EIM W external miswire fault

High severity

What the control is telling you

Voltage sensed on the outdoor W output when Y1 out is off; W is miswired. S30/S40 thermostat alert code 421 (shown as E421 on the unit control display).

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

Likely causes

  • W output wired incorrectly

Test and repair steps

  1. Correct the W output wiring.
  2. Clears when voltage is no longer sensed or power is cycled.

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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.32. 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.