Power

I recently bought a second hand Iseries machine.During the boot process, it shows the orange light with the error code 286C3203.
I don't know anything about the hardware of these devices.I only found little information that the problem possibly refers to the I/O P
card.

I would not have purchased such an old model.  The exception might be if I had a side gig supporting someone with such an old model and I purchased another as a backup.  

1)  If the reason you purchased it was for hardware education then what hardware information you learn from this old model will likely not transfer to knowledge of newer systems.

2)  If the reason you purchased it was for learning programming then many of the newer enhancements are not available on such an older model

3)  If it was for software knowledge you can get access to machines with current versions of the software via sites like https://pub400.com/  Not only is that site free but the energy costs of an 800 can be substantial.

I see a good response on another thread dealing with this same code suggesting card replacement, etc.  I would try them.  He just wasn't aware that you just recently purchased this system and thus would not have hardware maintenance on it.  Best of luck.