Lennox SLP98UHV Error Code E 124: Thermostat Signal Missing
What Does Code E 124 Mean?
On the Lennox SLP98UHV, E 124 means the active communicating thermostat signal has been absent for more than 3 minutes. The furnace has stopped all heating and cooling because it no longer receives instructions from the thermostat, and it will not resume until valid communication returns.
The SLP98UHV is an icomfort communicating system. Instead of a bundle of individual thermostat wires, it uses a four-wire communicating bus (R, i+, i-, C) over which the SureLight Variable Capacity Integrated Control and the thermostat exchange digital messages. The thermostat sends a steady stream of heartbeat data; when the SureLight board hears nothing on the bus for over 3 minutes, it logs E 124 and idles the equipment. Because the fault is about a specifically lost thermostat, it is related to but distinct from E 105 (the board finds no other devices at all on the communication bus) and E 120 (a device is present on the bus but stops responding to polling). E 124 specifically means the thermostat itself went silent.
The most common homeowner-fixable cause is a dead or weak thermostat battery. Many Lennox communicating thermostats rely on batteries as a power source, and when they drain the thermostat stops talking to the furnace. A thermostat that has locked up and needs a reboot is also common. Deeper causes — a loose, pinched, or damaged communicating wire, or a failed thermostat — require a technician. The code clears on its own the moment the board receives a valid message again.
What You'll Notice
- The thermostat screen is blank, frozen, or dimly lit and does not respond to touch
- No heating or cooling occurs even though the thermostat is set to call for it
- The furnace 7-segment display shows E 124 when you recall error codes
- The home slowly drifts cold in winter (or hot in summer) because the system sits idle
- The furnace makes no sound at all — the blower and burners never start
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 This Is Diagnosed
The cause is isolated from the thermostat outward: confirm the thermostat has power (fresh batteries or, if hardwired, a lit display), then reboot it, then verify the four communicating wires are seated. If the thermostat still cannot reach the furnace after that, the problem lies in the communicating wiring run or the thermostat hardware, which is where a technician takes over.
How to Fix It: Replace Thermostat Batteries and Power-Cycle
What You'll Need
- Replacement batteries (check the thermostat for type, typically AA or AAA)
- Small Phillips screwdriver (only if the thermostat is screwed to its base)
Steps
- Turn off electrical power at the breaker and shut off the gas supply valve Switch the furnace circuit breaker to OFF and turn the manual gas shutoff valve to the OFF position (handle perpendicular to the pipe) before you work on the system. If you smell gas, leave immediately and call your gas company.
- Replace the thermostat batteries Gently pull the thermostat off its wall base or open its battery door, and install fresh batteries of the correct type. Even a display that still lights up can have batteries too weak to keep the communicating link alive, so replace them rather than testing them.
- Power-cycle the thermostat With fresh batteries in, leave the thermostat off its base for about 30 seconds, then snap it back on. This forces a full reboot. If your thermostat has no batteries and is powered only through the furnace, it will reboot when you restore furnace power in the last step.
- Visually check that the communication wires are seated With the thermostat off the base, look at the wires landing on the terminals (commonly R, i+, i-, C). Confirm each one is fully seated and none are loose, corroded, or broken. Do not remove, swap, or rewire anything — just verify seating.
- Restore power and gas, then test Turn the gas valve back to ON (handle parallel to the pipe) and switch the breaker to ON. Reattach the thermostat if you removed it and wait 2-3 minutes for the thermostat and furnace to re-link, then set a heat call.
When to Call a Professional
Contact a licensed HVAC technician if:
- The thermostat display stays blank after fresh batteries are installed
- E 124 returns after replacing the batteries and power-cycling the thermostat
- A communicating wire at the thermostat or furnace looks damaged, pinched, or corroded
- The thermostat powers on but the furnace never responds to a heat or cool call
- E 105 or E 120 appears alongside E 124, pointing to a broader communication-bus fault
Frequently Asked Questions
Why does E 124 come back right after I replaced the batteries?
If the new batteries are weak, installed backwards, or the thermostat has not finished rebooting, the communicating link can still be missing. Give it a few minutes after reseating the thermostat; if it keeps returning, the wiring or thermostat likely needs a technician.
My thermostat is hardwired with no batteries — what causes E 124 then?
On a communicating Lennox system without batteries, E 124 usually points to a thermostat that has locked up or a loose or damaged communicating wire. A power-cycle may clear a lockup; if it does not, the wiring run or thermostat should be checked by a pro.
Is E 124 dangerous?
It is not an immediate safety hazard — the furnace simply stops because it has no instructions from the thermostat. The real consequence is that your home will not be heated or cooled until the communicating link is restored.
✓ Verified against manufacturer service manual — March 2026