Lennox Furnace Error Code E115

E115 indicates that the 24-volt control circuit voltage has dropped below 18 volts, which is outside the acceptable operating range of 18 to 30 volts. The furnace control board monitors this supply voltage and shuts down operation when it falls too low to ensure reliable control function. The error will self-clear and the furnace will attempt a restart once voltage recovers to an acceptable level. A technician should identify and correct the cause of the low voltage to prevent the error from recurring.

The E115 code itself reflects a control circuit voltage issue rather than a direct safety hazard such as a gas or carbon monoxide problem. The furnace shuts down to protect its controls from operating in an unreliable state. However, because the furnace will not heat properly while this error is active, the condition should be corrected promptly, especially in cold weather, to maintain safe indoor temperatures.

Repair costs depend on the root cause. If the fix is simply tightening a loose connection or removing an unnecessary accessory from the low-voltage circuit, service call fees alone may apply, typically in the range of 75 to 150 dollars. If a new transformer is needed, parts and labor combined generally range from 150 to 350 dollars, depending on the transformer VA rating required and local labor rates. Upgrading to a higher-capacity transformer or rewiring for additional zones may increase costs further.

The E115 error is designed to self-clear once the 24-volt supply voltage returns to within the 18 to 30 volt acceptable range, so a manual reset may temporarily clear the code. However, if the underlying cause of the voltage drop is not addressed, the error will return once the furnace attempts normal operation and the low-voltage circuit is again placed under load. A reset alone is not a repair.

Start by measuring the actual voltage at the 24-volt transformer secondary terminals and at the control board input using a multimeter. Check for any smart thermostats, humidistats, zone controllers, or other accessories recently added to the system that could be increasing the load on the transformer. If the transformer output is low under load, it may need to be replaced or upgraded to a higher VA rating. A licensed HVAC technician should perform this diagnosis to ensure accurate voltage measurements and safe component replacement.