The 8 series trucks can be diagnosed three ways depending on the unit.There is a method using the tilt lever and hour meter,a method using the optional display(more upgraded truck) and the Toyota plug in analyzer(hard to find and very expensive in my opinion). The truck can store 10 codes at a time. The problem you have ran into is a common problem. Its not receiving the return signal from the oxygen sensor to computer. The code you have can really only be three things 1. harness defect(broken wire or dirty connectors) 2.a failed o2 sensor. 3. a defective ecm. First i would check the wiring from o2 sensor to the ecm for any breaks or dirty connections,if any found, repair, then turn key to run position depress the accelerator pedal 5 full times within 30 seconds then start and code should be cleared