Lennox Error Code 65543: Firmware updated (Heat Pump)

Updated October 13, 2026 · by Aurora Star Tech Inc.

Lennox error code 65543 on a heat pump means: Firmware updated. It is also called Lennox diagnostic code 65543.

Low severityHeat Pump

Homeowner? What Lennox code 65543 means for you

Usually minor Your heat pump is reporting: Firmware updated. This code is usually informational or minor. The safe checks below often clear it; call us if it keeps showing up.

Safe things you can check yourself

Check the thermostat: right mode (Heat or Cool), set a few degrees past room temperature, and fresh batteries if it uses them.
Check both breakers and the outdoor disconnect box beside the outdoor unit; all should be ON.
Look at the air filter. If it is grey and clogged, replace it; a blocked filter causes many shutdowns.
In winter, keep snow and ice off and away from the outdoor unit. A short defrost cycle with steam is normal.
Try one reset: turn the power off for 30 seconds, then back on. If the code comes back, stop resetting and call a technician.

Call (647) 657-2977 Book a service call

Licensed technicians serving Vaughan, Toronto and the GTA.

What does Lennox error code 65543 mean?

New thermostat firmware installed successfully (information only). S30/S40 thermostat alert code 65543.

Applies to: Lennox communicating heat pumps incl. variable-capacity inverter units in S30/S40 smart thermostat systems

Common causes of Lennox code 65543

Firmware update

Safety: Gas, electrical and refrigerant work must be done by a licensed technician. Turn off power and gas before servicing, never bypass a safety control, and always follow the manufacturer's manual for your exact model.

Lennox models that use code 65543

EL16KP1-018 EL16KP1-024 EL16KP1-030 EL16KP1-036 EL16KP1-042 EL16KP1-048 EL16KP1-060 ML14KP1-018 ML14KP1-024 ML14KP1-030 ML14KP1-036 ML14KP1-042 ML14KP1-048 ML14KP1-060

Other Lennox Heat Pump codes

Code	Meaning
65537	Missing base (smart hub)
65538	Missing base
65539	Thermostat lost connection or internal fault
65540	Missing HD wall display
65541	Firmware download failed
65542	Firmware update failed
65544	More than 8 thermostats in a group
65545	Cooling capacity alert
70001	IAQ internal sensor fault
70002	IAQ internal data fault
70003	IAQ side switch in OFF position
70004	Replace Smart IAQ monitor

All Lennox Heat Pump error codes →

Lennox code 65543: homeowner questions

What does Lennox code 65543 mean?

Firmware updated. New thermostat firmware installed successfully (information only). S30/S40 thermostat alert code 65543.

Is it safe to keep using my heat pump with code 65543?

Usually yes. Code 65543 is normally minor, but if it keeps coming back have it checked.

Can I reset Lennox code 65543 myself?

You can try one reset (power off for 30 seconds, then on). If Lennox code 65543 comes back, the cause is still there and a licensed technician should check it.

Who can fix Lennox code 65543 near me?

Aurora Star Tech Inc. repairs Lennox heat pumps in Vaughan, Toronto and across the GTA. Call (647) 657-2977 or book a service call online.

Homeowner help: AC Running But Not Cooling: What to Check · Heat Pump Not Heating or Blowing Cold Air

Troubleshooting guides

Heat pump not defrosting / outdoor coil iced - Lennox EL16KP1 / ML14KP1 Lennox · Heat Pump
Low capacity or abnormal pressures - Lennox EL16KP1 / ML14KP1 Lennox · Heat Pump
Outdoor unit will not start - Lennox EL16KP1 / ML14KP1 Lennox · Heat Pump
Two horizontal lines on display (soft disable) - Lennox communicating Lennox · Heat Pump

Source: Lennox Residential Communicating Systems Alert Code Guide (V2) 100017 (3/2023), Table 2 Alert Codes and Troubleshooting, p.48. Written in our own words as a summary; always check the original manual.

General guidance for qualified technicians. The manufacturer's installation and service manual and local codes always take priority. Aurora Star Tech is not affiliated with, endorsed by or sponsored by any equipment manufacturer; brand and model names are used only to identify equipment and belong to their owners.