have it put onto IDS and using realtime display it will show which one is the bad one in a matter of mins.

alternativly unplug one, drive car see what happens, then replug and unplug the other one go for drive, which ever one is faulty once unplugged car should go into preset values, had this problem with missus racing puma car kept cutting out and it was the post cat one, once unplugged car ran fine, i did check it with realtime display first before spending any money on it though, new one cost me £70 from eurocar parts