The system displays F3; the compressor may not start or may short-cycle; this is sometimes accompanied by outdoor unit humming.

Safety first: Turn off power at the outdoor disconnect. Refrigerant leaks can be hazardous; avoid inhaling vapors.

Steps to address F3 (low-pressure switch open):

• Visual inspection: Look for oil stains on refrigerant lines, coil joints, and service valves, which indicate leaks.
• Check airflow: Ensure the indoor filter and coil are clean and the blower is operating correctly; poor airflow can cause low suction pressure.
• Inspect low-pressure switch: Locate the switch on the suction line. Check wiring and continuity. If the system is off and equalized but the switch remains open, replace it.
• Call a licensed technician: Low-pressure faults usually indicate a refrigerant leak or restriction. A certified HVAC technician must locate and repair leaks, evacuate, and recharge the system to factory specifications.

Do not attempt to add refrigerant without fixing leaks; this is illegal in many jurisdictions and will not provide a lasting repair.