It has never really been written out. I play the type of Paladin that confuses the DM and occasionally annoys the other players. Some of it is restrictions because he is a noble and a representive of his family. Most are because that is just who he is.
This is not a code for most paladins to follow, but here it is in no real order of importance.
Beliefs: I will not hold any other person but myself to my code or beliefs. Everyone has a right to choose their own actions and paths. But I will not follow another's lead on a path I believe to be wrong and will inform them to the reasons of my absence. I will seek to bring to proper justice a person who has broken the laws of our kingdom and seek incarceration for their crimes.
Mercy & Killing: Life is sacred. I will never take a humanoid life, opting to subdue even if it angers others. I will try my best to take as few animal lives as possible, since their nature is to hunt and they may have cubs to take care of. I will never strike a celestial until I have exhausted all possibilities of a nonviolent solution. Even then, I will only strike to subdue. Demons and Devils shall receive no mercy, but I will not be the aggressor in the fight. Undead are an abomination and should be destroyed, unless it will cause several political repercussions for the family. If it is the choice between destroying an obviously dangerous undead and political repercussions, I will destroy the creature and take full responsibility for my actions, trying to smooth out the problems. Since one cannot successfully subdue a creature with regenerative capabilities, I will strike to kill if they leave me no choice. I will not participate in a fight if I believe we are in the wrong and in which a comrade was the aggressor.
Truth: I will always tell the truth, for it is unbecoming of a man of my station to lie. When dealing with touchy political situations, I will either not be the speaker for my family, do my best to avoid topics of secrecy, or flat out tell them I cannot answer such questions.
Aid: I will render aid to anyone that needs it, so long as such actions will not lead me to break the laws of my kingdom. I will do my within the letter of the law to help them, however and I will work with the system to remove unjust laws.
Alignment: Alignment means nothing in this world of shades of gray. An evil man can be trusted as much as a good man. Either can betray a trust for their cause. I will be wary of those strongly devoted to their cause, for they are the easiest to convince to do the greatest evil. I will not forget that a clan of Paladins slaughtered mine fifty years ago because they were told we were devil worshippers. Trust can only be based upon a person's beliefs and the strength of those beliefs.
Money & Power: I will not seek power, for it is a corruptor. I will only seek to hone my abilities to protect others. I will be as little of a drain on the family's resources as possible. I will take only what I specifically need and use it for the protection of our clan and what is right.
I think that is all of it. Whew! That took a while to post and word.