What a Lennox Smart Thermostat Alert Code Means
Last updated: October 31, 2026 · Reviewed by Colton Air Conditioning, NV license 0092120
Reviewed by Colton Air Conditioning, NV license 0092120 Lennox communicating systems (iComfort S30, S40 and other Lennox smart thermostats) report problems as numbered alert codes. Lennox's own Residential Communicating Systems Alert Code Guide sorts each code by priority, and the priority tells you how urgent it is.
What the priority labels mean (per Lennox)
- Service Urgent: the system is in a no heat / no cool condition or not operating. Lennox says a dealer service call is needed to get it running. These show on the homeowner's thermostat screen.
- Service Soon: the system is not reaching set point or is only partly operating.
- Service Soon / Service Urgent: the system will likely recover on its own; after a set time or number of repeats, some escalate to Service Urgent.
- Maintenance: service reminder intervals set by your installing dealer.
- Information Only-Dealer: information for the dealer, not shown to the homeowner.
Common outdoor-unit codes, in Lennox's words
| Code | Lennox alert text | What Lennox says it means |
|---|---|---|
| 105 | Wiring Issue / Communication Problem | A communication problem between components. Lennox's steps start with wiring connections and grounding. |
| 400 | OD LSOM Comp. Internal Overload Tripped | The compressor's internal overload tripped: there is a cooling demand (Y1), but the compressor is not running. |
| 401 | OD Compressor Long Run Cycle | The compressor ran more than 18 hours straight on a single cooling demand, or refrigerant pressure is low. |
| 402 | OD System Pressure Trip | Discharge or suction pressure is out of limits, or the compressor overloaded. |
| 403 | OD Compressor Short-Cycling | The compressor ran less than three minutes to satisfy a thermostat demand. |
| 404 | OD Compressor Rotor Locked | Locked rotor, which Lennox lists as a run capacitor short, seized bearings or excessive liquid refrigerant. |
| 411 | OD Low Pressure Switch Strikes Lockout | The low pressure switch opened five times in one demand, and the thermostat shuts the outdoor unit down. |
| 413 | OD High Pressure Switch Strikes Lockout | The high pressure switch opened five times in one cooling demand, and the outdoor unit shuts down. |
| 414 | OD High Discharge Line Temperature | Discharge line temperature is above 279°F. |
Several of these (411, 413) clear only after the cause is corrected and the control is reset, per Lennox. Lennox's troubleshooting steps for them include checking refrigerant charge and pressures, which takes EPA-certified refrigerant work.
What you can safely do
- Note the exact code number and the alert text from the thermostat's notifications screen. That speeds up the visit.
- Don't keep clearing and restarting a lockout code. Lennox designed the strike lockouts to stop the outdoor unit after repeated faults.
- If a furnace is involved and you smell gas, leave the house and call Southwest Gas or 911 from outside.
Codes not listed here are left out on purpose: we only list codes we've checked against Lennox's own guide. Related: frozen AC coil Call (702) 738-6976 to schedule. If the code keeps coming back, it needs a diagnosis: see air conditioning repair across Las Vegas or AC service in Henderson, or Call (702) 738-6976 to schedule.
Sources
- Lennox, Residential Communicating Systems Alert Code Guide (V2), doc 100017, 3/2023: https://www.lennox.com/dA/d89d9db1dd/100017c.pdf (priority table, Table 2: codes 105, 400–404, 411, 413, 414)