Lennox Heat Pump Error Code Alternating FAST Flash (Green/Red)

This code means the control board has detected that the outdoor coil temperature sensor circuit is either open or shorted. The board reads sensor resistance continuously and compares it to a known temperature-resistance table. When the reading falls outside the expected range, defrost is disabled. The system will continue to heat and cool, but without defrost the outdoor coil can ice over in cold conditions.

The system will continue to operate, so there is no immediate safety hazard. However, without a functioning defrost cycle, the outdoor coil can accumulate ice in temperatures near or below freezing, which can stress the compressor and reduce heating output significantly. It is best to have the sensor issue diagnosed and repaired promptly, especially heading into cold weather.

Power cycling the unit may temporarily clear the code display, but if the underlying sensor or harness fault is still present, the board will detect the out-of-range resistance reading again and re-flag the fault quickly. A reset does not fix the hardware problem. The defrost lockout will remain in effect until the sensor circuit reads within the expected resistance range per Table 4.

A knowledgeable DIYer can inspect the sensor harness for visible damage, check that the connector is fully seated at the control board, and use a multimeter to measure sensor resistance against Table 4. If the harness is shorted or open, replacing it is a straightforward repair. However, if the sensor itself or the control board needs replacement, working with refrigerant-side components or high-voltage wiring should be left to a licensed technician.

Repair costs depend on what component has failed. A replacement coil sensor or harness typically costs between $15 and $80 in parts, and a technician visit to diagnose and replace the part usually runs between $100 and $250 in labor depending on your region. If the control board itself is faulty, board replacement can add $200 to $600 or more in parts and labor.