This air-conditioning system washer seals the joint between the compressor and its mounting flange or between the line connection and the evaporator housing. It prevents refrigerant and lubricant loss at the sealed interface. A degraded washer allows moisture and debris into the closed loop, which can accelerate compressor wear and reduce cooling efficiency. The manufacturer supplies this washer as an original-service component, so its thickness and durometer match the factory stack-up; using a substitute of different compressibility can alter clamp load and cause the joint to leak. On reinstall, clean both mating surfaces and torque the fastener to specification.