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. The 24V circuit powers the thermostat, gas valve, and control logic, so low voltage on this circuit can cause erratic operation or complete system shutdown.
The transformer inside the furnace converts the incoming line voltage (120V) to the 24V used by the control system. A failing transformer with weakened windings will produce lower voltage under load. The problem can also be caused by excessive demand on the 24V circuit — adding accessories like a whole-house humidifier, electronic air cleaner, UV light, or smart thermostat can overload a marginal transformer.
The control board will attempt to restart if the voltage recovers. However, operating on borderline voltage stresses electronic components and can cause intermittent problems. A technician should measure the transformer's output voltage under load, check for shorts or excessive draws on the 24V circuit, and replace the transformer if it is not delivering adequate voltage.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Failing transformer or excessive load on the 24V circuit | Most common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- The furnace is shutting down intermittently due to low 24V
- The transformer needs to be tested under load by a technician
- Multiple accessories are connected to the 24V circuit
- The thermostat display is dim or resetting on its own
✓ Verified against manufacturer service manual — March 2026