Samsung Air Conditioner Error Code P421

P421 is a low pressure protection fault. It activates when the refrigerant suction pressure drops below 0.7 kg/cm²G during cooling or below 0.2 kg/cm²G during heating. The unit shuts down to protect the compressor from running under dangerously low pressure conditions. This can be caused by closed service valves, a faulty PWM valve, a refrigerant leak, or a failing pressure sensor.

No. The P421 fault is a protective shutdown designed to prevent compressor damage from low refrigerant pressure. Bypassing or ignoring this fault and forcing the unit to run can result in compressor failure, which is the most expensive component in the system. Keep the unit off until the underlying cause is diagnosed and corrected.

Repair costs vary significantly depending on the cause. A simple service valve adjustment may cost only the technician's diagnostic fee, typically in the range of $75 to $150. PWM valve coil replacement or a sensor swap can add $100 to $300 in parts and labor. If a refrigerant leak is involved, locating and repairing the leak plus recharging refrigerant can bring total costs to $300 to $800 or more depending on the severity and refrigerant type.

One check you can perform yourself is confirming that all service valves on the outdoor unit are fully open, as a closed valve is a common cause of this fault. However, diagnosing refrigerant leaks, testing PWM valve coil resistance, and handling refrigerant require certified technicians with proper equipment. Attempting to add refrigerant without finding the root cause is unsafe and ineffective.

A power reset may temporarily clear the fault and allow the unit to restart, but P421 will return if the underlying pressure problem is not resolved. If the cause is a closed service valve, opening the valve and resetting may clear it permanently. If the cause is a refrigerant leak, faulty PWM valve, or sensor failure, the fault will reappear and the root issue must be addressed first.