Sometimes referred to as O2 or Lambda Sensor measures the amount or proportion of oxygen mixed in the fuel of your vehicle which is essential in the proper operation of your catalytic converter. The vehicle then determines whether it is necessary to adjust for it to perform properly. A lot of people buy the wrong oxygen sensor because of the wrong plug and length of the sensor. Please make sure to check your old sensor to see if the plug and pins are alike or better than the whole sensor itself. Compatible Downstream Sensor with the Following Vehicles: 2014-2017 Forester 2015-2017 WRX Replaces Part Numbers: 234-4988 234-5178 250-241160 250-25125 This is a compatible replacement part manufactured by AA Ignition. This 02 Oxygen Sensor is NOT a genuine OEM part and their associated vehicle models and part numbers are used in this listing only to identify the vehicles and part models this oxygen sensor fits.