The short answer
When a Lennox thermostat stops working in Northern Virginia, it’s usually one of four things: the screen is asleep, the thermostat lost power from the furnace, it can’t find the equipment after a power blip, or only the Wi-Fi is down. Touch the screen first. Then check the furnace switch, the breaker and the blower door. If the screen says System Under Test, turn the power to the system off and back on. If it’s still dead after that, the thermostat or its wiring needs a tech.
Blank screen: start here
Touch it. Lennox thermostats have a Power Save setting where the screen stays blank until it’s touched, and the M30’s default screen saver blanks the screen too. A screen that wakes up when you touch it isn’t broken.
Just put an S30 screen back on the wall? Give it three minutes. The S30’s display has a rechargeable battery that only charges on its wall base. Lennox says a low battery can keep the screen blank for up to three minutes after it’s docked.
Is the furnace on? The S30, M30 and S40 aren’t battery thermostats. They run on 24-volt power from the furnace or air handler; Lennox says the M30 needs a common wire to operate. So anything that kills power at the furnace kills the thermostat: the furnace switch off, a tripped breaker, or the blower door off its switch. Check all three.
On an SL280 or SLP99, check the control breaker. It’s a small 3-amp breaker in the control box with a button on its face. Press it once. If it trips again, a wire is shorting and a tech needs to find it. How to reset a Lennox furnace shows which buttons are safe to touch.
Any recent work? A new thermostat, a paint job or a contractor in the closet can pull a thin thermostat wire loose. If the screen died right after, say so when you call.
The $67 Heating Check
We check your furnace, heat pump or boiler before winter. Through December 31. See the offer
The S30 Smart Hub light tells you a lot
An iComfort S30 has two parts: the screen on the wall and a Smart Hub, usually mounted near the furnace or air handler. The hub’s status light is the quickest read on what’s wrong:
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| Smart Hub status light | What Lennox says it means |
|---|---|
| Solid green | No system errors |
| Red | An HVAC fault, a critical alert |
| Blinking amber | Wi-Fi not connected, out of range, or dropping in and out |
| Blinking magenta | The screen isn’t connected to the hub. The screen may be blank or say it can’t talk to the equipment. |
| Cyan | The indoor unit isn’t connected or can’t communicate |
Blinking magenta with a blank screen points at the wires between the hub and the display, or a display that isn’t seated straight on its base. Reseat the screen on the base. If the light keeps blinking, the communication and 12-volt wires at the hub need a tech with a meter.
It’s on, but it won’t do anything
System Under Test. Lennox says it usually shows after a power interruption, while the thermostat looks for the indoor and outdoor controls. The outdoor control sometimes boots slower and misses it. Recycle power to the system: furnace switch and outdoor breaker off, then on.
An alert on the screen. Photo it before you do anything. Lennox Service Urgent alerts and thermostat messages decode the common ones. If you need to reboot it, Lennox thermostat reset shows how without losing settings.
It works, but the app says offline. The thermostat is fine; the Wi-Fi link isn’t. Lennox’s own checklist:
- Use your secure home network. Lennox says never a guest network and never an open one.
- Keep the thermostat and router away from microwaves, baby monitors, cordless phones, wireless cameras and Bluetooth speakers.
- Check the signal on the thermostat’s Wi-Fi screen. Lennox calls -1 to -69 RSSI the ideal range.
- Two dashes where the outdoor temperature should be means the thermostat lost the internet or the Lennox server, or the account is logged out.
- On an M30, Lennox’s test is to connect it to a phone’s hot spot. If it reaches the Lennox server within five minutes, the thermostat’s Wi-Fi works and the router is the problem.
What it usually turns out to be
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| What we find | How a tech confirms it | Our price |
|---|---|---|
| Tripped control breaker or blown fuse from a shorted wire | Volts at the board and at the thermostat wires | Priced in writing |
| Loose or broken thermostat wire | Volts at the board but none at the wall | Priced in writing |
| S30 screen not talking to the hub | Communication and 12-volt wires checked at the hub | Priced in writing |
| Thermostat failed | Powered and wired right, still dead | $300 to $640 |
| Furnace control board | No 24 volts out of a powered board | $550 to $1,180 |
The diagnostic is a flat $99, and you see the repair price in writing before we start. If there’s no Lennox in the picture, thermostat not working covers every brand.
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. Other brands too: furnace and AC 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
Why is my Lennox thermostat screen blank?
Often it’s Power Save, which keeps the screen dark until you touch it. If touching it does nothing, the thermostat isn’t getting power from the furnace: check the furnace switch, the breaker and that the blower door is on.
Does a Lennox thermostat have batteries?
The S30, M30 and S40 run on 24-volt power from the furnace or air handler, not batteries. The S30’s screen has a rechargeable battery that only charges on its wall base, so a screen that was off the wall can take up to three minutes to wake.
Why does my Lennox thermostat say System Under Test?
It usually follows a power interruption, while the thermostat looks for the indoor and outdoor controls. Turning the power to the system off and back on often clears it.
My Lennox thermostat works but the app says offline. What now?
The thermostat is fine; the Wi-Fi link isn’t. Make sure it’s on your secure home network, not a guest network, and away from microwaves and baby monitors. Restarting the router is a fair first step.
