actually i misunderstood, i didnt remove the o2 sensor i just unplugged it (the frst oneunder the hood and not the second one.) It has been 5 days now driving with the o2 unplugged and the hesitation is completely disapeared but i did stall all of a sudden again while driving . I just lift the clutch while the car is in gear and moving to restart it. Is the o2 sensor that i unplugged no good (cuz i just bought it last month brand new- after market brand). Or is it really the cat? or is it the other o2 sensor? or is it fuel pump or something......

 

The o2 sensor is fine. it's the cat. P0420 is a cat code, not an o2 sensor code