Lennox furnace E110: Low line voltage
E110 means the incoming line voltage is below the furnace's nameplate rating. Lennox's action is to check and correct the supply; the alarm clears five seconds after the voltage recovers.
Applies to: Lennox gas furnaces with a seven-segment diagnostic display on the control board, such as the ML193UHEK, per installation instructions 508565-01.
What it means
This furnace measures its own supply voltage and reports on it, which most furnace controls do not. E110 says the line voltage is lower than the nameplate rating on the unit. Low voltage matters because motors draw more current to do the same work as voltage falls, so a furnace running on a sagging supply runs hotter and wears faster — the inducer and the blower motor most of all. The alarm clears five seconds after the fault recovers, which means an intermittent E110 in the code history points at something that comes and goes rather than a permanently undersized supply.
Most common causes
- A utility supply genuinely running low, which affects the whole house rather than the furnace alone.
- An undersized or long branch circuit feeding the furnace.
- A loose or corroded connection at the disconnect, breaker, or in the furnace junction box.
- A large load elsewhere in the house pulling the voltage down when it starts.
- A failing breaker or disconnect switch.
How to diagnose it
- Note whether lights elsewhere in the house dim, particularly when a large appliance starts.
- Note whether the code appears at particular times of day, which points at load rather than wiring.
- Note whether other equipment in the house has been behaving oddly.
- Voltage measurement at the furnace is technician work — it means opening a live cabinet.
How to fix it
- If the whole house is affected, this is the utility's problem before it is the furnace's.
- If only the furnace circuit is affected, the branch circuit, its connections and the disconnect need checking by an electrician or HVAC technician.
- The furnace resumes normal operation five seconds after the supply recovers, so nothing needs resetting.
- Do not ignore a recurring E110. Running motors on low voltage shortens their life measurably.
Notes
E110 and E113 are the two ends of the same measurement. Between them the furnace is effectively acting as a supply-voltage monitor, which is genuinely useful information that most furnaces do not provide.
Related
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
How low is low on this furnace
The 508565-01 manual puts a floor under the guesswork: in its generator-use section it specifies 120 volts ±10% for this furnace, meaning the control expects to see at least roughly 108 volts. It also caps the circuit's over-current protection at 15 amps and requires copper supply wiring — details that matter here because a furnace sharing a long, marginal branch circuit with other loads is exactly the installation that sags below that floor when something else starts. An E110 that appears the moment a well pump, air conditioner or workshop tool kicks on is describing the wiring between the panel and the furnace, not the furnace.
What the fix costs
The immediate bill is small: a diagnostic or service call at $75–150 to measure the supply at the furnace under load, then electrician time if the fault is a loose lug, a failing breaker, or an undersized branch circuit. What makes a recurring E110 expensive is ignoring it. Motors compensate for low voltage by drawing more current and running hotter, and the two motors in this furnace are the costly ones — a blower motor replacement runs $400–1,500 installed depending on motor type. A $150 electrical repair that stops the sag is cheap insurance against that invoice arriving early. More codes like this across brands: power supply codes.
DIY or call a pro
The observation work is yours; the meter work is not. For this specific code, useful homeowner evidence is a pattern: do the lights dim when the code appears, does it cluster at high-demand hours, does it coincide with one particular appliance starting? That distinguishes a whole-house supply problem (call the utility — checking their side costs nothing) from a furnace-circuit problem (call an electrician) before anyone is paid. Do not open the furnace's junction box or the disconnect to inspect connections — a connection loose enough to drop voltage is also hot enough to burn, and it is energized. And resist the urge to treat E110 as cosmetic because heat still works; the code clearing itself five seconds after each recovery is exactly why the underlying sag goes unfixed for years.
What this looks like
From the outside, this usually shows up as the furnace is completely dead — that walkthrough covers what to check in order, and when it stops being a homeowner job.
This can also show up as the furnace not turning on at all — that page is a 5-minute checklist before calling anyone.