Lennox Thermostat Alert Codes: S30, E30 and S40

By Ray Bennett · Updated 2 October 2026 · 5 min read · Checked against the maker’s manual

Lennox Thermostat Alert Codes: S30, E30 and S40

Short answer

Lennox's smart thermostats show an alert code with a priority. Service Urgent means the system is not heating or cooling and needs a dealer. Service Soon means a dealer within 24 to 48 hours. The 3000 codes are reminders you reset yourself. A few you can check: 371 is the float switch, 250 the furnace limit, often a dirty filter, and 29 an indoor temperature over 90°F.

Lennox’s communicating thermostats, the iComfort S30 and E30 and the newer S40, report faults from every part of the system as a numbered alert. Lennox publishes the whole list in its alert code guide, written for dealers, with a priority for each code and the way it clears.

Most codes need a dealer. This page sorts out which ones you can check yourself, which ones clear on their own, and what the priorities and colors mean.

Before you start

What to carry: a torch and a pen. Ten minutes.

Gas or electric? Both. Lennox’s codes cover gas furnaces, air handlers, air conditioners, heat pumps, zoning and the thermostat itself.

Read the notification. Lennox says selecting a notification on the thermostat shows a short description and the alert code. Write the number down before anything is reset.

Leave these to the dealer. Service Urgent. Lennox says the system is not heating or cooling, or not operating, and a dealer service call is needed.

Code 200, the rollout switch. Lennox tells the dealer to correct the cause of the trip and check the vent pipes and the flame: what a rollout switch is telling you.

Gas. If you smell gas, leave the house and call from outside before anything else.

What the priorities mean

Lennox's four notification types on the E30 and S30, by color. Red and orange mean call your service technician.

Check 1

Read the priority first

Service Urgent. The system is in no heat or no cool, or not operating, and needs a dealer service call.

Service Soon. Lennox says the system is not reaching its setpoint or is only partly working, and a dealer needs to service it within 24 to 48 hours.

Maintenance. Reminders for filters, UV lamps and tune ups.

The colors. Lennox’s user guide shows critical faults in red and warnings in orange, both meaning contact your service technician. Reminders are yellow and information is blue.

The codes you can check

Lennox alert codes that point at something an owner can look at. The rest need a dealer.

Check 2

Check the drain, the filter and the room

371, the float switch. Lennox’s code for an air handler float switch that has opened. After 10 minutes it moves from Service Soon to Service Urgent: clearing the drain line.

250, the furnace limit switch. Lennox lists low supply air from a dirty filter or blocked ductwork among the causes. If it does not close within three minutes, the furnace rests for 60 minutes: the limit switch.

29, over temperature. The thermostat reads over 90°F indoors and will not allow heating until it falls below that. Lennox says to check the heating is not stuck on and select cooling.

30, low temperature. The thermostat will not allow cooling until it senses more than 40°F. Lennox says to select heating to warm the space.

700, the thermostat’s sensor. Lennox suggests taking the display off its mount and refitting it, and sealing the hole in the wall behind the mount against cold air from inside the wall.

The codes that clear on their own

Check 3

Give it time, then watch

38 and 39, no heat or no cooling. After 45 minutes without heating or cooling, Lennox says the system goes offline for 60 minutes and then tries to restart itself.

403, short cycling. The compressor ran less than three minutes to satisfy a call. It clears after four normal compressor runs.

402, a pressure trip. A pressure out of limits, or a compressor overload. It also clears after four normal runs: why an AC short cycles.

600, load shed. Your utility has cycled the compressor off to limit demand. It clears when the utility’s signal ends.

601, too cold outside. Lennox says the outdoor unit will not operate when it is 4°F or colder outside. It clears when the cold passes.

The codes for a dealer

Code	Lennox’s alert text
105	Wiring issue or communication problem in the low voltage wiring
124	The thermostat lost communication with the smart hub for more than three minutes
200	Furnace rollout limit switch open
204	Check the furnace gas valve
270	Furnace flame failed to ignite, five tries in one call
400	Compressor internal overload tripped
401	Compressor ran over 18 hours in one cooling call, or refrigerant pressure is low
404 to 408	Compressor rotor locked, open circuits, or a welded contactor
444	Heat pump reversing valve relay or solenoid fault

Reminders and restarting

Check 4

Reset the reminders yourself

The 3000 codes. Lennox’s user guide lists 3000 and 3001 for filters 1 and 2, 3002 the humidifier pad, 3003 the UV light, 3004 a maintenance visit and 3005 PureAir maintenance.

Setting them. Lennox says reminders are off by default and can be set for 3, 6, 12 or 24 months, or a date you choose.

Restarting. Under Advanced Settings, Restart lets you restart the smart hub, the thermostat or both.

The filter behind the code. How often a filter really needs changing: how often to change the furnace filter.

Questions people ask

What does Lennox alert code 371 mean?

The air handler’s float switch has opened, usually because the condensate drain is backing up. After 10 minutes Lennox raises it to Service Urgent.

What does Lennox code 105 mean?

A wiring or communication problem in the low voltage wiring between system parts. It is a dealer job to trace.

What is the difference between Service Urgent and Service Soon?

Service Urgent means no heat or no cooling and a dealer call now. Service Soon means the system is only partly working and needs a dealer within 24 to 48 hours.

How do I clear a Lennox alert code?

Most clear on their own once the problem is gone. The 3000 maintenance reminders you reset yourself after the filter, pad or service is done.

Why won’t my Lennox heat pump run in very cold weather?

Code 601: Lennox says its outdoor unit will not operate at 4°F or colder. It clears when the temperature rises.

Why does my Lennox thermostat say over temperature protection?

Code 29: it reads above 90°F indoors and will not heat. Lennox says check the heating is not stuck on, and select cooling.

Sources: Lennox Residential Communicating Systems Alert Code Guide (V2), 100017, March 2023, and the Lennox iComfort E30 / S30 Smart Thermostats User Guide 507920-02