HVACopediaLennox · Air handler
E348 Indoor relay Y2 fault
What the control is telling you
The indoor control did not see 24 VAC after relaying a Y2 compressor demand. S30/S40 thermostat alert code 348 (shown as E348 on the unit control display).
Applies to: Lennox communicating air handlers and EIM-equipped indoor units in S30/S40 smart thermostat systems
Likely causes
- Y2 relay failed (contacts did not close or coil did not energize)
Test and repair steps
- Measure 24 VAC on Y2 output during a second-stage cooling call.
- If no voltage, replace the control.
- Clears automatically once fixed.
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Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.26. Summarized in our own words; check the original manual for your model.
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