Lennox · Furnace

E390 Low GWP relay stuck

High severity Lennox ML180UHE

What the control is telling you

The Low GWP relay on the furnace control is not working correctly. The control may need replacing. Clears when the relay operates normally.

Applies to: ML180UHEK (R-454B era) single-stage 80% furnaces with integrated control 107792-XX (ML180UH045/070/090/110/135, incl. low-NOx X models)

Likely causes

Failed Low GWP relay on the integrated control

Test and repair steps

Cycle power and run an LGWP test (short press of the test button) to see if the relay now operates.
Check nothing external is wired wrongly to the ALARM or ZONE dry contacts (rated 3 A at 30 VAC/DC).
If the code persists, replace the integrated control.

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Source: Lennox ML180UHEK Unit Information 100161/100166, Table 6 Integrated Diagnostic Codes, p.15. Summarized in our own words; check the original manual for your model.

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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.