1985 Nissan Truck Idle's rough/ no power

Hi I have a 1985 Nissan pickup with a 2.4l 4 cylinder engine. I was driving it the other day and all of a sudden it started running rough and lost power. I also noticed that when I brought the RMP's up it sounded like a rapping noise in the engine but it didn't do it at lower RPM's. I have replaced the Distributor Cap, Rotor Button, Plug Wires, Spark Plugs, Fuel Pump, Fuel Filter and Fuel Pump Control Unit. I have even pulled the valve cover and checked the valves and valve springs as well as the gap between the valves and the rocker arms. The engine idles very rough and I do have spark from both coils. I did notice that the timing chain has some play in it and I was wondering if it could have possibly jumped time. I also noticed the yellow sensor light is on on the dash. I'm not sure if this is an O2 sensor light indicator if so if it would cause the engine to act this way. I was also wondering how to retrieve any diagnostic codes from the vehicle and if there is a way that I can get access to what the codes would mean. Thanks.