There is no code for this that will consistently work on a stock PS2, however, there are some other ways to make some easy cash:

Probably the easiest way to gain some extra cash is to start a pink slip race, of any kind. As soon as you select the pink slip race remove your memory card. Hit OK to go to the race, you'll see a message telling you a memory card was not found and auto-save will be disabled - so go ahead and race. If you win put your memory card back in, then go to the Extras section of the Career screen and enable auto-save.

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