Short answer: There is no Max Motives cheat for the PS2 version of The Sims 2 Pets. Cheat codes—including the famous maxMotives—are exclusive to the PC release. The console port was built without a cheat console or developer command access.
Since you can't freeze motives, design a routine that keeps them high naturally:
The PS2 version prioritized pick-up-and-play simplicity. EA omitted the cheat console to prevent save corruption and maintain parity with the game's "intuitive" design philosophy. PC players got the full debug toolkit; console players got streamlined gameplay.
If maxed motives are essential, the PC version (with expansions) is the only legitimate route.