Lennox Error Code 31: Lost Communication with Device (Meaning, Causes, Fixes)
What the code means
In a Lennox communicating system (iComfort, S30, S40 and similar smart thermostats), the thermostat constantly exchanges messages with every connected Lennox component. Lennox's alert code guide defines alert 31, Lost Communication with Device, as the applicable component (indoor unit, equipment interface, damper control module or outdoor unit) not communicating with the thermostat for more than three minutes. It's rated Service Urgent. Lennox's troubleshooting steps are to check connections and voltages, ohm the wires for continuity, check whether a float switch on the air handler drain pan has been tripped by a clogged condensate line (it's wired in series with the R terminal, so a trip cuts 24 VAC power to the control), and check whether a freezestat is installed. If the fault persists, cycle power. The alert clears by itself once communication is restored. Older Lennox communicating thermostats (manual P506052-01) used the same code 31 for Lost Communication with a device. Related alerts are 105 (a device communication problem, often wiring or noise) and 124 (thermostat lost communication with the Smart Hub).
Likely causes
- A clogged condensate drain line that has tripped the float switch on the air handler drain pan, which is wired in series with R and cuts power to the control (Lennox names this specifically)
- An open freezestat (coil freeze protection switch) wired into the circuit
- Loose, corroded or broken low-voltage wiring between the thermostat (or Smart Hub) and the unit
- Loss of 24 VAC power at the component, for example a tripped breaker, a switched-off furnace or air handler, or a blown control fuse
- A failed control board on the component that stopped communicating
Safe DIY checks
Turn off power at the furnace/HVAC service switch and the breaker before opening panels.
- Note which device the thermostat says has lost communication (indoor, outdoor, equipment interface or zoning). That tells you where to look.
- Check that the furnace or air handler and the outdoor unit have power: breakers on, the furnace service switch on and the outdoor disconnect in place.
- Look at the air handler or furnace drain pan and condensate line. If there's water in the pan or the float switch is tripped, clear the clog (a wet/dry vacuum on the drain line outlet works well), let the float drop and the alert should clear.
- Check the condensate line again after the system has run a while, since a partly clogged line can trip the float switch repeatedly.
- With power off, check the low-voltage wire connections at the thermostat and at the unit's control board for loose, corroded or chewed wires.
- Cycle power, as Lennox recommends: turn off the indoor unit's power (which feeds 24 VAC to the system) for a minute, then back on, and wait several minutes for the thermostat to reconnect.
- If alert 31 comes back after the drain and wiring check out, the wires need continuity testing with a meter or a control may have failed, which is a job for a technician.
🛠 When to call a pro
- Alert 31 keeps returning after you've cleared the drain line and cycled power.
- The thermostat names the outdoor unit or zoning control and you can't find a power problem.
- You'd need to open the furnace or air handler cabinet to test 24 VAC or ohm the communication wires.
- The system is new or was recently rewired, since miswiring can cause communication alerts.
- A freezestat is tripping, which points to an airflow or refrigerant problem.
Parts that commonly fix it
Always match the part number to your exact model.
Wet/dry vacuum
The easiest way to pull a clog out of a condensate drain line from the outside end.
Search eBay →Digital multimeter
For checking 24 VAC at the control and testing wire continuity with power off.
Search eBay →HVAC air filter
A clogged filter can freeze the coil and open a freezestat. Match the size printed on your current filter.
Search eBay →Sources
- Lennox Residential Communicating Systems Alert Code Guide (V2, 100017), Table 2: alert 31 Lost Communication with Device (Service Urgent, more than three minutes, check connections and voltages, ohm wires, float switch in series with R, freezestat, cycle power), plus alerts 105 and 124
- Lennox communicating thermostat manual P506052-01, alarm codes: 31 Lost Communication with device (not communicating for more than 3 minutes, check connections, cycle power)
Last reviewed 2026-10-25. Always confirm against the manual for your exact model number.