how can i mend my 1998 vauxhall vectra?

i bought a 98 vauxhall vectra 1.8 16v but at the moment the engine managment light stays on and wont idle, it just cuts out.ive replaced the camshaft&crankshaft sensor,idle control valve and the air flow meter.ive cleaned the throttle body and cat.when we put the car on the diagnostic computer its reading a faulty o2 sensor so i replaced it but nothing still the same,yet the computer is still reading an 02 sensor fault.please please help!!!