Lennox Furnace Error Code E205

E205 means the furnace control board has detected 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 shuts down to protect itself and its components from operating under insufficient voltage. The control is designed to automatically restart if the voltage recovers on its own. A technician should check the transformer output and the 24-volt circuit to identify the root cause.

The furnace will not operate while E205 is active because the control board shuts down to prevent component damage from insufficient voltage. There is no immediate safety hazard from the error itself, but you should not attempt to bypass or defeat the protection. Leave the system off until a technician can inspect the transformer, wiring, and control circuit.

Repair costs vary depending on the cause. If the fix involves tightening connections or tracing a wiring fault, labor costs alone may be in the range of 75 to 150 dollars. Replacing a 24-volt transformer typically costs between 150 and 350 dollars including parts and labor. If the control board itself is faulty, replacement costs can range from 300 to 700 dollars or more depending on the specific board required.

E205 is a self-recovering error, so if the 24-volt voltage rises back into the acceptable range, the control board will attempt to restart automatically without a manual reset. However, if the underlying cause of the low voltage is not corrected, the error will return. A manual reset will not fix the root cause, so diagnosis of the transformer and control circuit is still necessary.

The first step is to measure the actual voltage at the transformer secondary terminals while the system is calling for heat, confirming whether it falls below 18 volts. If the transformer output is low under load, check for an overloaded circuit by disconnecting accessories and loads one at a time. A failing transformer will need to be replaced, and any shorted wiring or components contributing to excess current draw should also be repaired. This diagnostic work is best performed by a qualified HVAC technician.