Check Engine Light

I got the O2 Changed as per the dealer but after couple of months i got the Check Engine light again?What could be the Problem Now

You need to have the car code scanned and depending on year and model, Auto Zone or Advance may be able to do this for you and it is free.

Chances are you'll need a new catalytic converter. I just conquered the same dilemma, once you get a diagnostic done preferrably for free at Autozone you might want to either spend $1200 at the dealer or about $650 getting a "Miller OBDII" certified cat converter. You can buy them directly at Miller catalyzer company dot com. Or some similar name configuration. Beware of other web sites because they'll sell you the same one for from $749-$970. Also you'll want to read their advice on what to do before you spend for the cat converter because it could also be as simple as spark plugs. Oh, one other thing, there is a cat converter on both sides cylinder 1-4 is the right, and 5-8 is on the drivers side.