Lennox Heat Pump Error Code Green SLOW Flash / Red ON

This code indicates a discharge line temperature fault. The refrigerant discharge line exiting the compressor exceeded 300°F (148°C), triggering an automatic compressor shutdown. The compressor will remain off until the discharge line temperature falls below 225°F (107°C). The fault is a protective response to an underlying system problem, most commonly related to refrigerant charge or airflow.

The system has already shut down the compressor to protect itself, so it will not run normally in this condition. Attempting to force the unit back into operation without correcting the root cause risks permanent compressor damage. Leave the system in its protected state and schedule a service call promptly, especially during extreme weather when the heat pump is needed most.

Repair costs vary widely depending on the root cause. A refrigerant recharge typically ranges from $150 to $450 depending on refrigerant type and the amount needed. Coil cleaning or sensor replacement generally falls in the $100 to $300 range for parts and labor. If the compressor itself has failed, replacement costs can range from $1,000 to $2,500 or more depending on the unit and labor rates in your area.

Some preliminary checks are within reach of a knowledgeable homeowner, such as inspecting and replacing air filters, clearing debris from the outdoor coil, and confirming airflow from supply registers. However, verifying refrigerant charge requires manifold gauges and EPA certification to handle refrigerants, so charge-related diagnosis and repair must be performed by a licensed HVAC technician. Sensor replacement and compressor evaluation also require professional service.

The compressor will automatically attempt to restart once the discharge line temperature drops below 225°F (107°C), so a manual hard reset is not always necessary to clear the lockout. However, if the underlying cause such as low charge or restricted airflow is not corrected, the fault will return quickly after the restart. A reset without diagnosis and repair is not a solution and can lead to repeated thermal stress on the compressor.