HVACopediaLennox · Furnace
E390 Low GWP relay stuck
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.
Settings for these units
Filter sizes by cabinet width · Lennox ML180UHE
| Cabinet | Side return | Bottom return |
|---|---|---|
| A - 14-1/2 in. | 16 x 25 x 1 (1) | 14 x 25 x 1 (1) |
| B - 17-1/2 in. | 16 x 25 x 1 (1) | 16 x 25 x 1 (1) |
| C - 21 in. | 16 x 25 x 1 (1) | 20 x 25 x 1 (1) |
| D - 24-1/2 in. | 16 x 25 x 1 (2) | 24 x 25 x 1 (1) |
Filters are field supplied; check monthly.
Cabinet letter is the letter after the cooling size in the model number: A = 045E36AK, 070E36AK; B = 070E36BK, 090E48BK; C = 090E60CK, 110E60CK; D = 135E60DK.
Models that show this code
12 models in Lennox ML180UHE
Other codes on the same units
- E250 Limit switch circuit open
- E270 Soft lockout - no flame after max retries
- E271 Soft lockout - retries exceeded, pressure switch opened
- E272 Soft lockout - recycles exceeded, pressure switch opened
- E273 Soft lockout - recycles exceeded, flame failure
- E274 Soft lockout - limit opened
- E275 Soft lockout - flame out of sequence, now gone
- E290 Ignitor circuit fault
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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.
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.