Lennox Heat Pump Error Code Green FAST Flash / Red OFF

This flash pattern indicates a discharge sensor lockout. The control board detected five out-of-range readings from the discharge temperature sensor during a single Y1 demand cycle and locked out the system to protect the compressor. The fault can be caused by a faulty sensor, damaged wiring harness, or in some cases an actual extreme discharge temperature condition. The system will not restart automatically until the lockout is manually reset.

The system has already locked itself out, so the compressor is not running while this code is active. It is not safe to repeatedly reset the lockout and force the unit to run without diagnosing the root cause, because the discharge sensor is there to protect the compressor from damaging temperature extremes. Continued operation without a functioning sensor could result in compressor failure.

Resetting the board clears the current lockout and allows the system to run again, but it does not fix the underlying fault. If the discharge sensor or harness has not been repaired or replaced, the board will detect five more out-of-range readings during the next demand cycle and re-enter lockout. A permanent fix requires diagnosing and correcting the sensor circuit fault rather than relying on repeated resets.

You can reset the lockout by cycling the 24-volt control power to the board or by shorting the TEST pins on the control board for one to two seconds. Either method clears the five-strike count and allows the system to attempt operation again. Keep in mind that if the underlying sensor or wiring fault is not corrected, the board will accumulate five more strikes during the next demand cycle and return to lockout.

Repair costs vary depending on which component has failed. A sensor harness repair or connector cleaning may cost between $75 and $200 in labor. Replacing the discharge temperature sensor typically runs between $100 and $300 including parts and labor. A control board replacement, if required, can range from $400 to $800 or more depending on the model and labor rates in your area.