Lennox Heat Pump Error Code DS2:AlternatingFastFlash / DS1:AlternatingFastFlash

This code indicates a coil sensor problem on your Lennox heat pump. The control board has detected that the coil thermistor circuit is open, shorted, or returning a temperature reading outside the acceptable range. As a result, the board disables all defrost operation, both demand-based and time/temperature-based, until the fault is resolved. Diagnosis begins with a resistance check at pins 5 and 6 on the control board using the brown wire harness.

The unit may continue to run in heating mode, but without a functioning coil sensor the defrost cycle is completely disabled. This means ice can accumulate on the outdoor coil, reducing heating efficiency and potentially placing strain on the fan motor and compressor over time. Running the system for extended periods in this condition is not recommended, especially in cold or humid weather where frost buildup is likely.

Repair costs vary depending on what component has failed and local labor rates. Replacing the coil sensor harness or thermistor sensor is generally a straightforward repair, with parts costs typically in the range of $20 to $80 and total service call costs often falling between $100 and $300 depending on the technician and region. If the control board input circuit is found to be faulty, board replacement will increase the total cost considerably.

Start by measuring resistance at pins 5 and 6 on the control board, which connect to the coil sensor via the brown wire. If the reading shows zero ohms or infinite resistance, the harness is shorted or open and should be replaced. If the harness is intact but the sensor value is still out of range, the thermistor sensor itself needs replacement. In rare cases where both the harness and sensor test correctly, the control board input circuit may be at fault.

Resetting the unit will temporarily clear the fault display, but the code will return if the underlying sensor or wiring problem has not been corrected. The control board continuously monitors the coil sensor circuit during operation, so a failed thermistor or damaged harness will trigger the fault again shortly after restart. A permanent fix requires repairing or replacing the faulty component.