The short answer
A Service Urgent alert on a Lennox S30 or S40 thermostat in Northern Virginia means, in Lennox’s own words, a “no heat, no cool or no ventilation status.” Write down the alert number on the pop-up. Then check the two things you can: the condensate drain and the filter. If neither is it, that number tells a tech where to start, and we can usually narrow it down on the phone.
How Lennox sorts its alerts
Service Urgent. Shows on your thermostat. Something has stopped heating, cooling or moving air.
Maintenance. Shows on your thermostat. Reminders, like the filter.
Service Soon. In Lennox’s description, the system isn’t reaching set point or is only partly running, and it should get looked at within 24 to 48 hours. These go to the dealer, not to you, and the dealer alert setting is off by default. So a problem can build for weeks with nothing on your screen. Some Service Soon alerts turn into Service Urgent if they keep going.
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We check your furnace, heat pump or boiler before winter. Through December 31. See the offer
The alerts we see most
Swipe to see all columns →
Swipe to see all columns →
| Alert | What Lennox says | What to do |
|---|---|---|
| 31 | Lost communication with a device for more than 3 minutes | Check the drain. Lennox’s own tip: the condensate float switch is often wired in series with the thermostat’s power, so a full pan looks like a dead connection. |
| 603 | Air handler float switch sensed open | The drain pan is full. Becomes Service Urgent after 10 minutes. Clear the drain. |
| 105 | Communication problem | Wiring between the thermostat, furnace and outdoor unit. A tech. |
| 115 | Low secondary 24 volt power | Needs 18 to 30 volts. The transformer or something wired to it. |
| 38 | Not heating when requested for more than 45 minutes | Goes offline for 60 minutes, then retries. Check the furnace for its own code. |
| 71511 | Indoor duct or filter blockage | Lennox’s advice: replace the filter or clear the ducts. |
| 150 | Refrigerant leak detected | Can’t be cleared while the sensor detects refrigerant. Refrigerant leak alarm |
| 400 to 426 | Outdoor unit alerts | Lennox AC and heat pump codes |
Messages that look bad and aren’t
Will start soon. A built-in 5-minute delay before the equipment starts. Normal.
Defrosting. A heat pump melting the frost off its outdoor coil. Normal.
Ambient lockout. A heat pump holding off under a set outdoor temperature, the balance point, so the backup heat takes over. That’s a setting, not a fault.
System Under Test. Can show after a power interruption. Lennox’s fix is to cycle the power.
70003. An indoor air quality accessory’s side switch is off. The email that comes with it can wrongly say no heat or no cool.
The drain is the one to check first
Your furnace and AC both make water. It runs to a drain, sometimes through a small pump. When the drain clogs or the pump fails, a float switch cuts the 24 volt power so the water doesn’t end up on your floor or through a ceiling. On a Lennox with an S40, that often shows as 31 or 603. Look for water in the pan under the coil or a pump with water to the brim. Clearing it often brings everything back (clogged furnace drain). Our price to clear the drain and add a trap and switch is $175 to $420.
Your thermostat says to call your dealer
The S40 shows whatever dealer name and number got programmed in at the install. That’s the installer, not the only company that can help. Any licensed HVAC company can read the alert history and diagnose it.
If the S40 itself is the problem, a thermostat replacement runs $300 to $640 at our prices. The S40 only talks to Lennox communicating equipment, so on those systems it gets replaced with another one, not a generic thermostat (thermostat not working).
The diagnostic is a flat $99. Our heating check is $67 per system, through December 31: furnace, heat pump or boiler. On a furnace, for $229 (regularly $467) we also pull the blower wheel and clean it blade by blade.
Every Lennox code in one place: Lennox error codes.
Call or book a Lennox repair
Call (703) 997-9757. We answer day or night. Or book online and pick a time. The diagnostic is a flat $99, and you see the repair price in writing before we touch anything. Furnace running fine and you want to keep it that way? Book the $67 heating check from goswiftpro.com/cookiesoffer (book by November 30) and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276. More on Lennox repair in Northern Virginia.
Questions people ask
What does Service Urgent mean on a Lennox thermostat?
Lennox uses it for a no heat, no cool or no ventilation status. Note the alert number, check the condensate drain and the filter, and call if neither is the cause.
What does alert 31 mean on a Lennox S40?
Lost communication with a device for more than 3 minutes. Lennox points to the condensate float switch, which is often wired into the thermostat’s power, so a full drain pan can cause it.
How do I clear a Service Urgent alert on a Lennox S40?
Fix the cause and it clears. Dismissing the pop-up doesn’t fix anything, and some alerts, like a refrigerant leak, can’t be cleared while the problem is still there.
Why does my Lennox thermostat say System Under Test?
It can show after a power interruption. Lennox says to cycle the power. If it keeps coming back, have a technician look.
