Lennox SLP98UHV Error Code E 115: Low 24V

What Does Code E 115 Mean?

Error code E115 on the Lennox SLP98UHV indicates that the 24-volt low-voltage supply has fallen below the acceptable operating range of 18 to 30 volts. This 24V comes from the furnace transformer's secondary side and powers the thermostat, gas valve, and the SureLight control logic, so sagging voltage on this circuit can cause erratic operation or a complete shutdown.

The transformer inside the furnace steps the incoming line voltage down to the 24V the control system uses. A failing transformer with weakened windings will produce lower voltage under load, and the problem can also come from excessive demand on the 24V circuit — adding a whole-house humidifier, electronic air cleaner, UV light, or an extra communicating accessory can overload a marginal transformer. It is important to understand which side of the power system E115 refers to: E115 is a low-voltage (secondary/transformer side) fault, while the E110 low line voltage, E113 high line voltage, and E114 out-of-range line frequency codes describe problems with the incoming utility power feeding the furnace. E115 points inward at the transformer and 24V circuit, not at the utility supply.

The control will attempt to restart if the voltage recovers, so the furnace may cut out and come back on its own. Running on borderline voltage stresses electronic components and produces intermittent faults. A technician should measure the transformer's output under load, look for shorts or excessive draws on the 24V circuit, and replace the transformer if it cannot deliver adequate voltage.

What You'll Notice

• E 115 shown on the furnace control's 7-segment display or in the error code recall menu
• The furnace resets or cuts out intermittently, then restarts a short time later
• The iComfort thermostat display looks dim, flickers, or resets on its own
• Symptoms get worse when an accessory such as a humidifier or air cleaner energizes
• The furnace runs briefly on a heat call, then drops out under load

Common Causes

How This Is Diagnosed

Because E115 clears itself when the voltage recovers, a homeowner can safely start by cycling power at the breaker to see whether the code was a one-time event; if it comes right back, the fault is persistent. A technician then measures the transformer's secondary output voltage both at rest and under load, since a weak transformer can read acceptable when idle but sag below range once the gas valve and blower draw current.

From there, the technician looks for what is pulling the 24V circuit down — a shorted wire, a failing gas valve or relay, or too many accessories wired to one transformer. If the wiring and loads are within limits but the output is still low, the transformer itself is failing and is replaced. This work involves live low-voltage testing and is not a homeowner task.

When to Call a Professional

This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:

• The code returns after cycling power at the breaker
• The furnace is shutting down intermittently and restarting on its own
• The transformer needs to be tested under load by a technician
• Several accessories such as a humidifier, air cleaner, or UV light share the 24V circuit
• The thermostat display is dim or resetting

✓ Verified against manufacturer service manual — March 2026