cheshou

Subaru Forester 1999 , 10900miles, P0420 Check engine light is on. Sometimes, it is off by itself, but it is on again after several times driving. the code is P0420. I cannot find any trouble in driving. So, my friend told me maybe the rear O2 sencor can not work effectively. Do your guys think he is right or have better idea? I have to pass the emission test in a month. So, thanks very much.