Lennox Heat Pump Error Code Alternating FAST Flash

The Alternating FAST Flash code indicates a coil sensor problem on your Lennox heat pump. The control board has detected that the outdoor coil temperature sensor is open, shorted, or returning a value outside the expected temperature range. As a result, the system disables demand and time/temperature defrost operation, though heating and cooling functions continue normally.

The system will continue to heat and cool with this fault active, so there is no immediate safety hazard. However, the defrost cycle is disabled, which means ice can accumulate on the outdoor coil during cold weather heating operation. Prolonged ice buildup restricts airflow, reduces efficiency, and can stress the compressor over time. The fault should be repaired in a reasonable timeframe.

Repair costs vary depending on the failed component. A coil sensor or sensor harness replacement is typically a lower-cost repair, with parts and labor often ranging from around $100 to $300 depending on your region and service rates. If the control board requires replacement, costs can range from $300 to $700 or more. Getting a diagnosis from a qualified technician before authorizing parts replacement helps avoid unnecessary expense.

Start by measuring resistance across pins 5 and 6 on the control board, which connect to the brown sensor harness wires. If the reading is open (infinite resistance) or shorted (near zero ohms), inspect the harness for damage or corrosion and replace the sensor harness if a fault is found. If the wiring is intact, the coil temperature sensor itself likely needs replacement. If both components test normally, have a technician evaluate the control board.

A power reset may temporarily clear the code, but if the underlying sensor or wiring fault is still present, the board will detect the problem again and re-display the fault. Resetting is useful for confirming whether the fault is intermittent, but it does not substitute for diagnosing and correcting the root cause at pins 5 and 6.