HVACopedia

Lennox · Heat pump

E406 Compressor open start circuit

High severity

What the control is telling you

Not enough current through the START current transformer. S30/S40 thermostat alert code 406 (shown as E406 on the unit control display).

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

Likely causes

  • Open start winding or wiring
  • Failed start components

Test and repair steps

  1. Check start circuit wiring through the current transformer and compressor start winding.
  2. Goes away on a power reset, or once the START sensor sees current.

Settings for these units

Save it for the next call

Star this code in the HVACopedia app and keep the whole library on your phone for jobs with no signal.

Open the app

Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.29. 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.