Puzzle items (Doom 3)

On certain maps in Doom 3 the player is required to collect a puzzle item which they need to use in order to progress (such as to unlock a door, or activate a mechanism). The player can hold one puzzle item at a time and it is shown in inventory in PDA. The puzzle item will be removed from the inventory upon using in it.

The puzzle items can roughly be divided into keycards and other items.

ACO keycard
The ACO keycard is used to access some offices in Mars City. When the player revisits the area after the invasion, they need to go through Conference Room which is locked by this keycard. It is found on a table in Reception Processing which is located nearby and happened to be unlocked.

Maintenance key 4
In Alpha Labs - Sector 3 the player encounters a locked door on their way and needs the maintenance key 4 to unlock it. The key is located in Hazardous Waste Disposal, but the room is contaminated by toxic gasses coming from two of the barrels. The player needs to remove those barrels by using a manipulator in a nearby control room to access. The key will be lying on the floor in the center of the room.

BRD access card
This card is used to access the Bio Research Division in Delta Labs - Level 2. Fortunately, just like ACO keycard, it is located not far from the door it unlocks, in Teleport Observation, on a bench next to a zombie that pretends to be dead.

Caverns access
This card is used to unlock a door in Sub Station 2 part of Caverns - Area 1, which the player has to go through. This card is given to the player by a survivor hiding in a maintenance shaft at the bottom of Thermal Regulation Pump.

Storage room 11 key
In Caverns - Area 2 the player can find this keycard on a table with a skeleton in Main Excavation. It is the only optional puzzle item in the game, which the player can use to open a storage room at the start of the level.

Administration
This key is found in the case with double barrel shotgun in Sarge's office in Erebus - Level 3 to open the door to the nearby Administrative Office, which contains one of power cells the player is looking for.

Armory Access
This key is given to the player in a cutscene by Dr. Cloud in Erebus - Level 5, after the cutscene where Cloud finishes analysing The Primitive. It gains the player access to Armory near the start of the level, which the player has to proceed through.

SkyTram Access
The player needs to find this key in order to unlock the door to SkyTram Station North in Phobos Labs - Sector 1, and it is found in Utility Closet.

Pumping Station
The key to Pumping Station is given in a cutscene to the player by Elizabeth McNeil in Phobos Labs - Sector 2. The player must use it to access and shut down the Pumping Station.

Reactor Key
To access Reactor Control in Phobos Labs - Sector 3 the player needs to find this key, which is located on a table in the lowest floor of Upper Maintenance, which itself is accessible from the lowest floor of Reactor Core.

Key card
An unnamed keycard is required to access Security Station in the beginning of Underground. It is located in Transfer Control, the same room that the locked door is in, but on the upper floor, to access which the player has to traverse through the entirety of the first part of the level.

Data linker
When player reaches Delta Labs - Level 1, they need to restart the reactor and restore the power in the complex to proceed, but to be able to access the reactor they need to gain the control access to the doors in this level, and to do this they need to retrieve the data linker. The data linker is located not far from control center, in Delta 1 Access Lobby, next to a body of its former owner.

Plasma inducer
In Delta Labs - Level 2 Ian McCormick asks the player to retrieve the plasma inducer to power up the teleporter so that the player can proceed further into the complex. This plasma inducer is located in the power room of Delta Operations, to which the player needs to gain access first.

DLib replacement panel
In Central Processing one of the doors the player has to go through is missing its control panel, so the player will need to find the replacement. The replacement panel is located on the third floor of Lab A in an opened storage crate.

Power cell
The power cell (named Pwr cell in-game) is an energy canister used to power up the AM3 series portable power plants, which are used to turn on the lights and electricity in an area around it. The player sometimes has to remove the cell from one power plant and put it into the other one to progress through the level. In some cases the player has to power up a power plant that requires two power cells instead of one, in this case the player needs to collect two cells on the level. Unlike other puzzle items, which are unique, the power cells appear on multiple levels in the expansion, and often several times in the single level.

The power cell also makes an appearance in The Lost Mission.

The Primitive
The Primitive is a fragment of one of the ancient stone tablets found in Site 1. According to Dr. McNeil, it serves as a decyphering tool of sorts. In Erebus - Level 3 she asks him to retrieve it from Artifact Storage and take it to Dr. Cloud in Level 5 for him to study.