Lennox Heat Pump Error Code DS2:FastFlash / DS1:OFF

This code indicates a discharge sensor lockout. The control board recorded five out-of-range faults from the discharge line temperature sensor during a single Y1 demand cycle and locked out the system to protect the compressor. The DS2 LED flashes rapidly while DS1 remains off to signal this specific fault condition. The underlying cause is typically a failed sensor, damaged harness wiring, or a poor electrical connection at the sensor connector or board.

Continuing to reset and operate the unit without correcting the sensor fault is not recommended. The discharge sensor is a primary protection input for the compressor, and running without reliable discharge temperature data risks undetected overheating. Repeated compressor overtemperature events can cause premature compressor failure, which is a significantly more expensive repair than replacing a sensor or harness.

A knowledgeable homeowner can safely perform the board reset procedure by cycling the 24V thermostat power and can visually inspect the sensor harness for obvious damage or loose connectors. However, accurately testing sensor resistance against temperature specifications and working near refrigerant lines and high-voltage compressor terminals requires proper tools and training. Full diagnosis and any component replacement is best handled by a qualified HVAC technician.

A reset, either by cycling 24V control power or by shorting the TEST pins for 1 to 2 seconds, will clear the lockout and allow the unit to restart. However, if the discharge sensor or its wiring is not repaired or replaced, the board will accumulate five new range faults during the next demand cycle and lock out again. The reset only clears the stored fault; it does not fix the root cause.

If the fix is limited to reseating a connector or repairing a chafed wire, the cost is primarily labor, typically in the range of $75 to $200 depending on local rates. Replacing the discharge temperature sensor itself usually adds $20 to $80 for the part. If the control board's sensor input circuit has failed, board replacement can push the total repair cost into the $300 to $700 range including parts and labor.