Lennox AC & Heat Pump Error Code E400: Meaning & How to Fix It

By the ErrorCodePro Editorial Team · Last reviewed October 20, 2026

Lennox ac & heat pump error code E400 means: There's a cooling demand but the compressor isn't running. Lennox says to check power to the outdoor unit. [1] Keep using it? Cooling is off until it clears. Start with the owner checks below, beginning with: check outdoor power.

Quick facts

Code	E400
Also shown as	None listed by the manufacturer
Brand / appliance	Lennox ac & heat pump (outdoor unit alerts (lsom) shown on icomfort-communicating sl280dfv systems)
System	Power supply
Official meaning	There's a cooling demand but the compressor isn't running. Lennox says to check power to the outdoor unit. [1]
Keep using it?	Cooling is off until it clears
DIY-safe?	Owner checks only (listed below)
Support	Lennox dealer (lennox.com)

On this page: Meaning · Steps · Parts · When to call a technician · Related codes · FAQ

What error code E400 means on an Lennox ac & heat pump

There's a cooling demand but the compressor isn't running. Lennox says to check power to the outdoor unit. [1]

This is a power supply code. Codes and wording can vary by model, so check your owner's manual for your model number.

How to fix Lennox ac & heat pump error code E400 (safe owner checks)

Owner checks only: on heating, cooling and water-heating equipment, limit yourself to reading the code and checking the thermostat, breaker and service switch, air filter, condensate drain, that the gas supply valve is open (no disassembly) and that vent terminals are clear of snow or debris. Reset power only if your manual allows it.

Safety first: disconnect power before you remove any panel or reach into the sump or drum. Do not touch wiring, the control board or the motor. If you see water near electrical parts, stop and call a technician. These steps are based on Lennox's published guidance [1], rewritten for clarity.

Check outdoor power. Make sure the outdoor unit's breaker hasn't tripped and the disconnect box next to the unit is switched on.
Check the thermostat. Make sure it's set to COOL (or HEAT for heat pumps) with fresh batteries.

Parts that may be involved

Exact parts depend on your model number. This page lists part types only, never part numbers.

No specific part is implied by the manufacturer's guidance for this code.

When to call a technician

Refrigerant charge, pressure switches, compressor, contactor, capacitor, sensors or wiring: EPA 608-certified HVAC technician
Gas valves, burners, igniters, flame sensors, venting/flue or CO concerns: licensed HVAC technician or plumber
Refrigerant work: EPA 608-certified technician only
Smell of gas: leave and call the gas utility or 911
Anything involving wiring, the control board, the motor or the water valve electrics
The appliance is under warranty: contact Lennox dealer (lennox.com) first

Frequently asked questions

What does error code E400 mean on an Lennox ac & heat pump?

Lennox lists E400 as follows: There's a cooling demand but the compressor isn't running. Lennox says to check power to the outdoor unit.

Is it safe to keep using the ac & heat pump with E400?

Cooling is off until it clears.

Can I fix Lennox ac & heat pump error E400 myself?

Only with the owner checks on this page, starting with: check outdoor power. If it persists, call a technician or Lennox dealer (lennox.com).

Sources

https://tech.lennoxintl.com/C03e7o14l/VIu12Ch2uV/1117n.pdf (official)

Last reviewed: October 20, 2026. ErrorCodePro is not affiliated with Lennox.