Lennox EL296UHV Error Code E 124: Thermostat Signal Missing
What Does Code E 124 Mean?
Error code E124 on the Lennox EL296UHV indicates that the furnace has not received a valid signal from the communicating thermostat (subnet controller) for more than 3 minutes. When this occurs, the furnace stops all heating and cooling services and waits for the thermostat to re-establish communication.
The EL296UHV uses a communicating system where the thermostat sends periodic heartbeat messages to the furnace control board. When these messages stop arriving, the furnace assumes the thermostat has failed or been disconnected and shuts down all services as a safety measure. The fault clears automatically once a valid thermostat message is received.
The most common cause is a loose or disconnected communication wire between the thermostat and the furnace. The four-wire communication cable can work loose from terminals at either end due to vibration or thermal cycling. A thermostat power failure (dead batteries on battery-powered models, or lost common wire power) can also cause the heartbeat messages to stop. Try cycling power at the thermostat first before calling for service.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Thermostat communication wire loose or disconnected | Most common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- The furnace shows E124 and cycling thermostat power does not resolve it
- The thermostat display is blank or unresponsive
- Communication wiring between the thermostat and furnace may be damaged
✓ Verified against manufacturer service manual — March 2026