HVACopediaLennox · Furnace
E124 Lost communication with thermostat
What the control is telling you
Communicating thermostat signal missing for more than 3 minutes; all services stop until a heartbeat is received.
Applies to: EL296UHV(X) series furnaces (integrated control 103130-XX and 107045-XX)
Likely causes
- Broken or loose thermostat wiring
- Thermostat (subnet controller) not powered or locked up
Test and repair steps
- Check all four thermostat wiring connections at the thermostat and the control.
- Ohm-check the thermostat wires for opens or shorts.
- Cycle power at the thermostat.
- Code clears when a valid thermostat message is received.
Settings for these units
Recommended filter size by cabinet width · EL296UHV
| Cabinet width | Side return | Bottom return |
|---|---|---|
| 17-1/2 in. | 16 x 25 x 1 in. (1) | 16 x 25 x 1 in. (1) |
| 21 in. | 16 x 25 x 1 in. (1) | 20 x 25 x 1 in. (1) |
| 24-1/2 in. | 16 x 25 x 1 in. (2) | 24 x 25 x 1 in. (1) |
Number in brackets is the quantity of filters.
Filters are field supplied and installed outside the furnace; inspect monthly.
Recommended filter size by cabinet width · Lennox SL280UHV
| Cabinet width | Side return | Bottom return |
|---|---|---|
| 14-1/2 in. | 16 x 25 x 1 in. (1) | 14 x 25 x 1 in. (1) |
| 17-1/2 in. | 16 x 25 x 1 in. (1) | 16 x 25 x 1 in. (1) |
| 21 in. | 16 x 25 x 1 in. (1) | 20 x 25 x 1 in. (1) |
| 24-1/2 in. | 16 x 25 x 1 in. (2) | 24 x 25 x 1 in. (1) |
Number in brackets is filter quantity; filters are field supplied.
Never run the unit without a filter installed.
- Filter sizes by cabinet width · Lennox SL297NV
Models that show this code
22 models in EL296UHV, Lennox SL280UHV, Lennox SL297NV
Other codes on the same units
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Source: Lennox EL296UHV(X) Unit Information / Service Literature, Table 5 Ignition Control 103130-XX Diagnostic Codes, p.34; Ignition Control 107045-XX Diagnostic Codes table, p.41. 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.