Lennox EL296UHV Error Code E 124: Thermostat Signal Missing
What Does Code E 124 Mean?
Code E124 on the Lennox EL296UHV means the furnace has gone more than three minutes without a valid message from the communicating icomfort thermostat. On this system the thermostat acts as the subnet controller and sends periodic heartbeat messages across the RS-Bus to the furnace and outdoor unit. When those messages stop arriving, the SureLight control assumes the thermostat is gone and halts all heating and cooling services until a valid message returns.
The design is deliberately conservative: because the thermostat is what directs the furnace's two heating stages and variable-speed blower, the control will not run blind. The fault clears automatically the moment a valid thermostat heartbeat is received again, which is why a simple power cycle at the thermostat often brings the whole system back.
The most common cause is a loose or disconnected communicating wire between the thermostat and the furnace, since the low-voltage conductors can work free from a terminal over time. A thermostat that has lost its own power — dead batteries on battery-powered setups, or a lost common-wire feed — will also stop sending its heartbeat. A homeowner can safely cycle power at the thermostat to test the easy cases; if that fails, the wiring must be inspected and ohmed by a technician. E124 is closely related to E105 (no devices found at all on the bus) and E120 (a specific device slow to respond); E124 is the one that points specifically at the thermostat's heartbeat going silent.
What You'll Notice
- All heating and cooling stop, and the furnace shows "E 124" on its LED
- The icomfort thermostat screen is blank, frozen, or shows a lost-connection message
- The system went dead after roughly three minutes of the thermostat being unresponsive
- Cycling power at the thermostat sometimes brings the system back
- The problem followed thermostat, wiring, or common-wire work
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Thermostat communication wire loose or disconnected | Most common | ✗ Call a pro → |
How This Is Diagnosed
Because a homeowner can safely cycle power at the thermostat, that is the first thing tried — if the thermostat was simply hung or lost power briefly, restoring it lets the heartbeat resume and E124 clears. If the thermostat is dead on battery-powered units, replacing batteries or restoring the common-wire feed is checked next.
When power cycling does not help, a technician inspects the communicating wiring end to end: confirming the conductors are landed and seated at both the thermostat and the furnace, and ohming the wires to find a break, short, or intermittent connection. Since E124 clears on the first valid heartbeat, verification is confirming the thermostat rejoins the bus and the system runs normally.
When to Call a Professional
This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:
- Cycling power at the thermostat does not clear E124
- The thermostat display stays blank or unresponsive
- The communicating wiring between thermostat and furnace may be loose or damaged
- E124 keeps recurring, or E105 or E120 appear along with it
Frequently Asked Questions
Can I do anything myself when I see E124?
Yes — it is safe to cycle power at the thermostat or at the furnace breaker to see if the connection re-establishes. If the thermostat runs on batteries, replacing them is also worth trying before calling for service.
Why does the whole system shut down over a thermostat signal?
The communicating thermostat is what commands the furnace's stages and blower. When its heartbeat goes missing for more than three minutes, the control stops rather than run without direction, and it resumes the instant a valid message returns.
How is E124 different from E105?
E124 specifically means the thermostat's heartbeat went silent, while E105 is the broader case where the control finds no devices at all on the RS-Bus. A technician uses which code appears to narrow down where the communication is breaking.
✓ Verified against manufacturer service manual — March 2026