Lennox SLP98UHV Error Code E 124: Thermostat Signal Missing
What Does Code E 124 Mean?
Error code E124 on the Lennox SLP98UHV indicates that the active communicating thermostat signal has been missing for more than 3 minutes. The furnace has lost its connection with the thermostat and has stopped all heating and cooling services until communication is restored.
The SLP98UHV uses Lennox's communicating protocol to exchange data with the thermostat (subnet controller). The thermostat regularly sends heartbeat messages to the furnace. When these messages stop arriving for more than 3 minutes, the control board triggers E124 and halts all operations as a safety measure — without thermostat instructions, the furnace does not know when to heat or cool.
The most common cause is a dead thermostat battery. Many Lennox communicating thermostats use batteries as a backup or primary power source. When the batteries die, the thermostat goes silent. Other causes include a loose or damaged communication wire between the thermostat and furnace, a thermostat that has locked up and needs a power cycle, or a failed thermostat. The error clears automatically once the furnace receives a valid message from the thermostat.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Thermostat lost power or has a dead battery | Most common | ✓ DIY fix → |
| Loose or damaged communication wire between thermostat and furnace | Common | ✗ Call a pro → |
How to Fix It: Replace Thermostat Batteries and Power Cycle
What You'll Need
- Replacement batteries (check thermostat model for type, typically AA or AAA)
- Small Phillips screwdriver (if needed to open thermostat)
Steps
- Turn off electrical power at the breaker and shut off the gas supply valve Locate the furnace circuit breaker and flip it to OFF. Turn the gas shutoff valve to the OFF position (handle perpendicular to the pipe). This ensures safety while you work on the system.
- Check and replace thermostat batteries Pull the thermostat off its wall plate or open its battery compartment. Replace all batteries with fresh ones of the correct type. Even if the thermostat display appears to be working, weak batteries can cause intermittent communication failures.
- Power cycle the thermostat With fresh batteries installed, remove the thermostat from the wall plate for 30 seconds, then reattach it. This forces a full reboot. If your thermostat does not use batteries and is powered only through the furnace wiring, it will reset when you restore furnace power.
- Check the thermostat wiring connections With the thermostat removed from the wall plate, visually inspect the four communication wires. Make sure each wire is securely seated in its terminal and none are loose, frayed, or broken. Do not rearrange or swap wires.
- Restore power and gas, then test Turn the gas supply valve to ON (handle parallel to the pipe) and flip the circuit breaker to ON. Reattach the thermostat to the wall plate if removed. Wait 2-3 minutes for the thermostat and furnace to re-establish communication. Set the thermostat to call for heat.
When to Call a Professional
Contact a licensed HVAC technician if:
- The thermostat display remains blank after replacing batteries
- E124 returns after replacing batteries and power cycling
- You notice damaged or loose wiring at the thermostat wall plate
- The thermostat powers on but the furnace does not respond to heating calls
- Multiple error codes appear alongside E124
✓ Verified against manufacturer service manual — March 2026