Finally the problem is resolved after opening the catalyst converter (i am attaching a reference picture).. after discussing with few mechanics most of them suggested to open it and clean it. so finally get it opened from the mechanic and cleaned thoroughly with acid (bathroom saaf krnay wala) 6 bottles.. and then washed it with clean water.. the pick has increased and with ac its way better then before..
P.S dont put screw driver or any sharp thing inside the converter