Mil Light Low Effeciency Bank 1 Help Please

[sporty1]

Hi my fusion 2004 as just passed an mot 2 a week ago , today the MIL light as come on and when i put a OBD CODE READER it comes up low effeciency bank 1 ( i forgot to write the code down )

what i need to know is could it just be a sensor OR WORSE CASE SENERIO IT BE THE cat EVEN THOUGH IT JUST PASSED MOT EMMISIONS ETC

with it saying bank 1 is this refering to the sensor at the very top of the exhaust were it goes into the manifold or the very bottom underneath the cat, and if i have to replace the sensor are both sensors the same part ( are they just a simple take out and relace or do i have to reset anything when i put a new one in ?

thanks