To enter a cheat code, bring up your phone by pressing Up, then Up again to access the keypad. (You must first do a couple of quests for Roman to get a phone this works on). Once a code has been entered correctly a new Cheats menu selection is available on your phone, below Options, where you can access the cheats without having to enter the phone number every time. NOTE! Some achievements will be blocked when using the cheats.
Get Full Health and Weapons:
Call GTA-555-0100 (482-555-0100) on your phone. (Blocks the Cleaned the Mean Streets achievement)
Call GUN-555-0150 (486-555-0150) on your phone. (Blocks the Cleaned the Mean Streets achievement)
Get Advanced Weapons:
Call GUN-555-0100 (486-555-0100) on your phone. (Blocks the Cleaned the Mean Streets achievement)
Get Full Health:
Call DOC-555-0100 (362-555-0100) on your phone. (Blocks the Cleaned the Mean Streets, Finish Him, One Man Army & Walk Free achievements)
Wanted Level Down:
Call COP-555-0100 (267-555-0100) on your phone. (Blocks the One Man Army & Mean Streets achievements)
Wanted Level Up:
Call COP-555-0150 (267-555-0150) on your phone.
Call HOT-555-0100 (468-555-0100) on your phone.
Call CAR-555-0142 (227-555-0142) on your phone.
Spawn FBI Buffalo:
Call CAR-555-0100 (227-555-0100) on your phone.
Call CAR-555-0147 (227-555-0147) on your phone.
Call CAR-555-0175 (227-555-0175) on your phone.
Call CAR-555-0168 (227-555-0168) on your phone.
Call MBK-555-0100 (625-555-0100) on your phone.
Call MBK-555-0150 (625-555-0150) on your phone.
Spawn Police Chopper:
Call FLY-555-0100 (359-555-0100) on your phone. (Blocks the One Man Army & Walk Free achievements)
Call WET-555-0100 (938-555-0100) on your phone.
To get 100% you have to complete the following:
* Complete all Main Missions.
* Complete all 10 Drug Deliveries.
* Complete all 10 Races.
* Complete all 10 Assassination missions.
* Complete all 30 car thefts.
* Collect all 10 cars for the Exotic Exports mission.
* Unlock all five friend's special abilities and complete the activities with Little Jacob, Brucie and Packie.
* Beat the computer or an online friend in all activities.
* Find all Random Characters and complete their missions (except for Jeff, Cherise, Clarence & Ivan).
* Complete all 30 Most Wanted Missions.
* Complete all 20 Vigilante Missions.
* Kill all 200 Pigeons.
* Complete all 50 Unique Jumps.
If you go to an Internet café and enters the URL www.whattheydonotwantyout oknow.com you will get to a page with maps of all weapon pickups, health, armor, vehicle locations, unique jumps and pigeon locations.
If you call 911 on your phone you can select if you need an ambulance, police car or a fire truck. Make your selection and wait for them to arrive so you can steal the vehicle. If you get into the ambulance you will get a small health boost and if you get into the police car you will get 5 shotgun shells.
Get Claude's outfit:
If you choose the kill Playboy X you will take over his safehouse. If you change clothes in that safehouse you can choose to wear the outfit Claude (the main character in GTA III) was wearing. Black jacket, green pants and black/white shoes.
Get to Happiness Island. Run up to the base of the statue to find a shirt on the ground. Walk over it to put it on. It has a picture of the statue and the text Happiness is... on the front and the text ...land on the back.
The heart of the Statue:
Get a helicopter and fly to Happiness Island. Jump out of the helicopter at the top of the statue's base so you'll land by it's feet. You can not take the stairs this high. Look at the sign next to the door. It says No Hidden Content Here. Enter the door and climb the ladder inside the statue. You will find a big, red, beating heart held in place by chains.
Head inside the old, abandoned Casino where the last mission takes place, at the north end of Alderney. Look at one of the trash piles on the ground to find a purple *** sticking out.
Across the road from Bank of Liberty you can find a house covered in graffitti. Some of the artwork is from old GTA titles.
Across the street from the museum in Algonquin you can find a portrait painter. The portraits are of old GTA characters.
Old GTA character's names graffitti:
Go inside one of the pink houses near Manny's community center. Go to the back of the inside and look at the wall. You will find graffitti with names from a lot of old GTA characters, like Claude, Tommy, Toni, etc.
OG Loc CD:
In your Bohan safehouse you can find a CD by OG Loc from San Andreas on the floor.
Golf Course Land Marks:
At the mini-golf course at Firefly Island you can find a small version of the giant chicken from San Andreas, the lighthouse from the GTA III version of Liberty City, the Watts Towers from Los Santos and the Ocean View Hotel from GTA: Vice City.
Easy One Man Army achievement:
To get the One Man Army achivement steal or spawn a helicopter and fly over an island that you don't have access to yet. It will raise your wanted level to 5 stars. Fly as high as you can and you'll be flying higher than the police helicopters, then just fly for 5 minutes.
Car repair glitch:
When your car breaks down and it wont start, get your phone up and type 911. As soon as your call gets through your car will start up again. (This also works for any other number including pre-set ones i.e. Roman's). It works for every vehicle.
Easy Gobble Gobble Achievement:
Go to the bowling alley that you took Michelle on a date to. Choose the left most lane. Move Niko to the left 2 full steps. Throw the ball slowly, you want the speed somewhere around 20 but it doesn't need to be exact. Do this three times.
Easy Walk Free Achievement:
Go to the northern most road in Bohan, the small island where you get a safehouse after your first one gets burned down. There is a point where the elevated train tracks come down and go into a tunnel. Stand beside that place and shoot people and cops until you get 4 stars. Then jump or drive over the edge and land on the train tracks below and head into the tunnel. Just keep going until you are out of the search radius and you lose you wanted level. But to do this you will need to have unlocked the island of Algonquin (the middle one) or else you'll get 6 stars for crossing under the river.
Easy One Man Army Achievement:
At the start of the game drive north to Bohan, the island where you get a safehouse after your first one is burned down. At the very northern most road there is a point where the subway train tracks go from an elevated line to underground. Drive a car from the road onto the tracks and head into the tunnel. Eventually you'll cross the river and end up on the locked island of Algonquin and receive 6 stars. Stay in the tunnel for 5 minutes and you'll get the achievement.
HOPE THIS HELPS :)