We thought it was only possible for Gameshark owners, but now, thanks to edisoncarter we have a jetpack unlimited height code for Action Replay (PAL).
|Jetpack Unlimited Height ARMax PAL:|
To tell once you have reached the maximum height, push forward and back on your PS2 controller. If they no longer work as they should, you have reached the maximum height. Press R2 or L2 to move from side to side at a steady level. To fly above the maximum height, press and hold L3, whilst pushing the X button.
This code is useful for those interested in the hidden interiors such as Liberty City, which is now accessible thanks to this code (for ARMax owners).
Related Link: GTAForums Topic