Lennox Furnace Code E124
Warning
The furnace has not heard from the communicating thermostat for more than three minutes. Equipment has lost communication with the thermostat.
Manual wording: “Active communicating thermostat signal missing for more than 3 minutes. Equipment lost communication with the thermostat. Check connections and cycle power on the thermostat.”
Documented for: SLP98UHV series and other Lennox communicating furnaces
Likely causes, most common first
- Loose or broken thermostat wiring
- Thermostat has lost power or locked up
- Communication bus connection problem
What you can safely check yourself
- Check that the thermostat screen is on and set to Heat with the setpoint above room temperature
- Cycle power on the thermostat as the manual directs
- Turn the furnace power switch off for 30 seconds and back on once
When to call a technician
If the code persists after a thermostat power cycle, the communication wiring or thermostat needs to be checked by a technician.
A Lennox furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- SLP98UHV Service Literature (Corp. 1029-L7), Table 14 - LED 7 Segment Status / Error Code and Integrated Control Diagnostic Codes (Lennox Industries Inc., SLP98UHV series variable-capacity communicating gas furnace (66,000-132,000 Btuh)) · checked 2026-08-25