on the cat GC25K i still need a serial number so i can look it up in the appropriate service docs but to answer your question about codes.
There are no code numbers so to speak, just a flashing ! light in some cases or in these trucks, the ! light just tells you there is a problem usually accompanied by the corresponding system light, and at this point you have to find out what is giving the problem by either using the cat software and hooking up to the ecm and seeing what is out of range or doing it the hard way, manually checking components.
Fortunatly even doing it manually isnt so bad, you check the alternator output voltage, check the alternator belt, check the fuel vac solenoid (FCSV), and make sure it is working, usually hear it rattle when engine is running, it pulses real fast if it is working correctly, it is located over where the lp converter is on a bracket mounted to the lp converter bracket, has vac hoses hooked to it. Check the accelerator pedal sensor and throttle body voltages.

Also check the O2 sensor and you can also clean out the lp mixer, sometimes a dirty mixer can cause problems that will set off the ! light, the air mixture ports get blocked and cannot mix the air/fuel correctly and it will run rich or lean causing the O2 sensor to malfunction by over compensating out of range.
Just remember if you clean out the mixer with carb cleaner or anything else like it make sure you blow it out thoroughly, the heavy vapors from this could damage the O2 sensor. Once you clean it start the engine and run it wide open till it clears out... usually a minute or so should do it.

Testing the Throttle body:
- start engine and accelerate above 1200rpm for 10 secs, then allow to return to idle.
- disconnect the TB harness , the rpm should drop from idle 750 to about 500.
- if the rpm does not drop remove the TB side cover and inspect springs, if springs are intact then perform chassis harness test on ecu pin 12, blue/black wire and pin 8 ground black wire. Key on should read 0.77v, engine idle (in neutral) should read 3.5v, full engine rpm should read 5.0v.
With the engine at full rpm, in neutral and at hydraulic main relief, the voltage at pin 12 at ecu should read 5.0 to 6.5v. If either wire is open the engine will only achieve idle speed.

Test the O2 Sensor:
- observe the oxy sensor voltage, 0.2 to 0.9v is the desired range.
If the O2 sensor voltage is 0.9v or above, the dash light will come on indicating a system problem. The error code will be for O2 sensor. This is normally caused by an incorrect dwell reading with the resulting problem being in LP mixer or throttle body. NOTE: rarely is O2 sensor.