Lennox Furnace Code E115: Low 24 VAC Causes & Fixes
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Lennox furnace code E115: what it means
Lennox Code E115 indicates low 24 VAC at the furnace control board — the low-voltage power that runs the thermostat, relays, gas valve and safety switches has dropped below the level the board needs to operate reliably.
The two documented fault categories are an overloaded (or failing) transformer and a low-voltage wiring problem. The 24-volt “control circuit” is fed by, but separate from, the 120 V line power supplying the furnace — line-side problems are flagged by their own codes, such as E110 and E111. When the control voltage itself sags, the board flags E115 rather than trying to run with unreliable power.
On most Lennox integrated control boards, E115 clears automatically once healthy 24 VAC is restored. If the underlying fault comes and goes, the code will keep reappearing. Because the legend can differ slightly between board revisions, always confirm the code against the diagnostic label inside the blower compartment door.
Lennox furnace code E115: common causes, ranked by probability
- Overloaded transformer — too much load on the 24 VAC circuit (added accessories, a stuck relay, or a component drawing more current than the transformer can supply) pulls the voltage down.
- Failing transformer — an aging or partially shorted transformer that can no longer hold 24 VAC under load.
- Shorted or grounded low-voltage wiring — a pinched, chafed or rodent-damaged thermostat or accessory wire creating a partial short.
- Loose or corroded low-voltage connections — poor terminations at the board, thermostat, or accessories causing voltage drop.
- A faulty 24 VAC accessory — humidifier, EAC (electronic air cleaner), or zoning panel drawing excess current or shorting the control circuit.
How to fix Lennox furnace code E115: safe checks first
These are the only steps a homeowner should attempt:
- Thermostat: Make sure it’s set to Heat and calling for heat. If it’s battery-powered and the screen is dim or blank, install fresh batteries.
- Air filter: A clogged filter won’t cause E115 directly, but replace a dirty one while you’re troubleshooting so airflow isn’t a compounding issue.
- Breaker/switch: Confirm the furnace breaker is on and the furnace service switch (the light-switch-looking box near the unit) is on.
- One reset: Turn the furnace switch or breaker off, wait 30 seconds, and back on — a single power cycle. If E115 returns, stop and call a pro.
- Visible vents/registers: Confirm supply and return vents are open and unobstructed.
- Condensate line (high-efficiency units): Make sure the drain isn’t backed up.
Do not open the cabinet, test wiring, or touch the transformer — all of that is technician work.
How a technician will diagnose it
A qualified tech will:
- Measure the 24 VAC output at the transformer and at the board terminals under load with a multimeter, comparing readings to spec.
- Isolate accessories (humidifier, EAC, zone panel) one at a time to see if removing a load restores normal voltage — this pinpoints an overloaded circuit.
- Inspect low-voltage wiring end to end for shorts, pinch points, corrosion, and loose terminations at the board, thermostat, and any accessories.
- Check the transformer for overheating or internal short, and confirm its VA rating is adequate for the total connected load.
- Verify the control-circuit fuse (if fitted) and confirm the board is receiving and passing proper voltage.
- Re-power and confirm E115 clears and stays clear under a full heating cycle.
If a quote jumps straight to a new control board without any voltage or wiring measurements, ask for the readings that justify it.
Symptom, cause and what to do
| Symptom | Likely cause | DIY action | Technician job |
|---|---|---|---|
| E115 with dim/dead thermostat | Low 24 VAC / dead batteries | Replace thermostat batteries; single power cycle | Measure transformer output; trace low-voltage supply |
| E115 returns after reset | Overloaded or failing transformer | Stop resetting; call a pro | Load-test and replace transformer; verify VA rating |
| E115 started after adding humidifier/air cleaner | Accessory overloading 24 VAC | None | Isolate accessory; add/replace transformer if needed |
| E115 intermittent, comes and goes | Loose or chafed low-voltage wire | None | Inspect and repair wiring and terminations |
| E115 plus furnace won’t run at all | Shorted control circuit | Confirm breaker/switch on | Locate and clear short; repair damaged wire |
Lennox furnace code E115 repair cost
Honest US ranges (parts + labor; regional variation applies):
- Single power-cycle reset (no fault returns): $0 DIY.
- Diagnostic/service call: $90 – $180.
- Low-voltage wiring repair (splice, re-terminate, replace a run): $120 – $300.
- Transformer replacement: $150 – $350.
- Adding a second transformer or higher-VA unit for accessory load: $200 – $400.
- Control board replacement (if the board is confirmed faulty): $400 – $1,200+.
Most E115 calls resolve on the lower end — a transformer or wiring fix — rather than a full board swap.
Related codes
- Lennox Furnace Code E110: Low Line Voltage Fixes & Costs — low 120 V line power (not the 24 V control circuit).
- Lennox Furnace Code E111: Reversed Polarity Fixes & Costs — line-voltage wiring polarity.
- Lennox Furnace Code E112: Poor/No Ground Causes & Fixes — an inadequate or missing equipment ground at the furnace.
- Lennox Furnace Code E113: Line-Voltage Frequency Out of Range Fixes & Costs — supply frequency distorted or outside the accepted range.
- Lennox Furnace Code 200: Causes, Fixes & Costs
Frequently asked questions
Can I fix Lennox Code E115 myself?
You can cycle power once and confirm your thermostat, breaker and filter are fine, but transformer and wiring work belongs to a licensed technician.
Does Code E115 reset on its own?
On most Lennox boards E115 clears automatically once stable 24 VAC returns, but if the fault repeats it will keep re-flashing until the cause is fixed.
What causes low 24 VAC on a Lennox furnace?
A failing or overloaded transformer, a shorted accessory, or a pinched or corroded low-voltage wire are the documented triggers for this code.
Is it safe to keep running the furnace with Code E115?
No. Low control voltage can cause erratic operation and further component damage, so have it diagnosed before relying on the system.