Lennox HVAC Error Code 105

Code 105 means the iComfort thermostat has lost communication with the air handler for more than 3 minutes. The thermostat expects to receive periodic heartbeat messages from the air handler, and when those stop arriving, it triggers this fault. All associated HVAC operations are suspended until communication is restored. The most common causes are wiring problems or a control board issue at the air handler.

A power cycle can clear code 105 if the fault was caused by a temporary communication glitch or software lockup. However, if the underlying problem is a wiring fault or failed control board, the code will return once the system attempts to re-establish communication. The fault should be fully diagnosed before assuming a reset has resolved it.

Repair costs depend on the root cause. If the issue is only a loose wire connection, a service call fee in the range of $75 to $150 may be the primary expense. Replacing a failed air handler control board can cost anywhere from $300 to $700 or more depending on the specific board and labor rates. Thermostat replacement, if required, typically falls in the $200 to $500 range for parts and labor.

Some causes of code 105 can be addressed by a knowledgeable homeowner. Start by cycling power to both the thermostat and the air handler, then check that all visible low-voltage wiring connections are firmly seated. If those steps do not clear the fault, diagnosing wire resistance and control board issues typically requires a multimeter and familiarity with HVAC wiring, so a certified technician may be needed.

Code 105 itself is not a safety hazard in the way that a gas or carbon monoxide fault would be. However, because the alarm stops all heating and cooling operations, it can become a comfort or safety concern in very hot or very cold conditions, particularly for vulnerable occupants. Address the fault promptly to restore normal system operation.