Features and Benefits: Fits: ROBAC1234-4, ROBAC1234-6 and ROBAC1234-9; Replacement Oxygen SensorReplacement oxygen sensor for Robinair R-1234yf machines and refrigerant identifiers. Your 1234yf machine or identifier should provide advance notice when the oxygen sensor is nearing the end of its lifespan. Oxygen sensor replacement is required to return machine to normal operation. If the oxygen sensor expires, neither the integrated identifier, nor the machine, is able to recover or add refrigerant to the internal tank.