The best thing to do is grab a OBD2 reader and connect it to the OBD2 port. That will give you your P code and tell you if you are running rich or lean. As the person above mentioned, check for vacuum leaks and see if the manifold is cracked. I know on the LPG systems, we were having a problem with the injectors and throttle body.

Your truck is staying in closed loop. With a code 42 you must have a gasoline system. Usually it isn't a failed oxygen sensor. If you don't have the diagnoser software I would check for vacuum leaks and fuel pressure. I have had cracked intake manifolds and dirty mass air flow sensors cause the code. You can clean the sensor. If you post your email there is some information I could send you.