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.

Cat GC25K, how can I clear the maintance light and is there a way to get the code

we could be of some help if you had specific model information on each unit.
Like Admin says we cannot share manuals here but we can try to help you if we have enough information.

from your description though
the one with no power?
i would remove the dash panel and check the key switch plug and see if it is burned up, these units have alot of lights on them that pull alot of amps and combi runs these light circuits directly off the accessory circuit of key switch that is not good enough to handle all these amps so connector burns up.
also check the fuses inside that cover just beside the keyswitch.
There is also a battery disconnect switch located on the side panel under where the driver sits i think, make sure it is in the 'connected' position and not turned off.

Now as for the steering problem on the other unit
these units are notorious for getting out of sequence when the transverse cylinder gets out of alignment from internal leakage. It is located inside the right outrigger frame.

usually you can steer unit back and forth in side drive mode several times and it should fix this for a while.
if it continues to happen frequently though that cylinder may need repairing.
if you are having any other wheel turning issues then i would check the magnetic sensor switches at each wheel and make sure they are working correctly, they will have an orange indicator light on them to let you know when they are on or off.
Each wheel should have them, and depending on what model there will most likely be multiple sensors so look very carefully, follow the wire leads to them so you can locate them, sometimes they are hidden inside a cover or where you can't see them very well.

From Admin....thank you for using the Forums but please do not ask for Manuals. Please state your problem and ask for help. Forklift manuals are almost all considered "proprietary information". The only proper place to get them is from your local dealer who is the legal representative of the manufacturer. We all need to respect the intellectual property rights of the copyright holders.

Our Forum Readers can answer specific questions about particular concerns, and even quote small sections of the manuals (it's called "fair use" to intelligently discuss the particular concern), but cannot be seen as trying to get around the rights of the copyright holders (usually the manufacturer).
So if you have a particular problem we are sure there are folks on the Forums here who can give you very well considered opinions about those concerns.

Thanks
Admin