Lennox Error Codes & Diagnostics
Complete diagnostic directory for official Lennox fault codes. Verify root causes, safe troubleshooting steps, and replacement part numbers before calling a technician.
Lennox iComfort alerts 10, 31 and 39 that keep coming back: tracing intermittent faults without swapping parts
Only one of the three is a communication code. Alert 31 reports that a component has not communicated with the thermostat for more than three minutes, and Lennox documents it as clearing once communication is restored. Alert 10 reports that an unknown device was found on the bus and needs configuring, not repairing. Alert 39 is a consequence: the system could not turn cooling on for more than 45 minutes, so it takes itself off-line for 60 minutes and retries. Replacing a control to chase 39 replaces the wrong thing. Applies to: Lennox communicating systems on iComfort S30, E30 and compatible displays, covering furnaces, air handlers, outdoor units, equipment interface modules and iHarmony zoning These entries come from the Lennox residential communicating systems alert table used with iComfort S30 and E30 displays. Lennox equipment running on a conventional 24 volt thermostat does not use this table, and legacy control-board flash patterns on the same appliance carry unrelated meanings. Owner-safe checks: Record the date, time and outdoor conditions each time an alert appears, and keep the list rather than clearing it. | Look for standing water or a slow drain at the indoor unit, because a float switch in series with the R terminal interrupts communication when it lifts. | Photograph any seven-segment display on the equipment, including a double horizontal line, before power is interrupted. Qualified technician: Voltage and continuity measurement at a communicating control is energised low-voltage work for a qualified technician. | Reconfiguration through the installer control center, and any isolation of devices on the bus one at a time, belongs to the installing contractor.
Lennox XP17 iComfort Critical Alert 411
For the Lennox XP17, Critical Alert 411 records repeated low-pressure-switch events that reached a lockout threshold. It does not prove low refrigerant charge: airflow, outdoor conditions, restriction, switch or wiring faults, and the refrigerant circuit all require operating measurements. Applies to: XP17 heat pumps with iComfort communicating controls This meaning follows the XP17 installation and service procedure cited here. Lennox alert numbers depend on the communicating equipment and control generation, so confirm the outdoor model and alert text together. Owner-safe checks: Save the complete iComfort alert history, operating mode, filter condition, weather, visible outdoor clearance, and recent service without opening equipment. Qualified technician: Pressure-switch, wiring, refrigerant, airflow commissioning, control-log, and live electrical diagnosis require a Lennox-qualified technician.
Lennox XP17 alerts 418 and 419: a W input fault or a miswire
Alert 418 is raised when the heat pump control sees the W output active while a compressor demand is running, which the installation manual describes as a W input fault or a miswire. Five occurrences within a single demand escalate it to a hardware fault lockout and alert 419. Applies to: XP17 heat pumps with iComfort communicating controls These alert numbers and the strike behaviour come from the Lennox XP17 installation and service procedure for the models listed. Lennox uses the same numbering family across several iComfort products, and the sequence a given alert follows belongs to the equipment it was published for. Owner-safe checks: Record the alert number, whether it was 418 or 419, and whether the system was still heating when it appeared. | Note any thermostat replacement, system conversion, or wiring work carried out before the alert started. | Note whether the property has a backup or auxiliary heat source and how it is meant to be controlled. Qualified technician: Low voltage wiring, thermostat configuration, and the field test pins are all inside equipment covers and belong to a qualified technician. | Ask for the wiring and the thermostat configuration to be verified before any part is quoted, since the manual attributes this alert to an input fault or a miswire.
Lennox iComfort keeps restarting: separating a control reboot from an equipment fault
A Lennox iComfort control that restarts on its own is describing its own operating conditions rather than a heating or cooling fault. What the control needs is a stable supply from the equipment to stay running, and a supply that dips, a loose conductor, or a software condition can all produce a reboot that looks alarming and means something specific. Applies to: Lennox iComfort communicating thermostats and the equipment controls they are paired with This covers Lennox iComfort communicating controls paired with compatible Lennox equipment. A Lennox system using a conventional thermostat behaves differently, and a restart on a non-Lennox control fitted to Lennox equipment is outside what Lennox documents. Identify the control model from its own settings menu. Owner-safe checks: Keep a short log of each restart with the time and what the heating or cooling system was doing in that moment. | Note which settings survive a restart and which have to be entered again. | Check whether any recent electrical work, network change, or equipment service preceded the behaviour. Qualified technician: Inspecting control wiring at the equipment, measuring supply under load, and updating control firmware are dealer tasks on a communicating system. | Do not add or move conductors at the control or the equipment to try to stabilise the supply. Communicating systems use those conductors for data as well as power.