Removing locks from the S5 that secure it during launch and lock it in place, relocating a radiator from its launch position to the bottom of the S5 to provide clearance for solar arrays to track the sun. During mission STS-120, the radiator will be relocated to its final position.

Moving a failed control moment gyroscope (CMG-3) to an unpressurized platform that holds spare parts outside the station called External Stowage Platform–2 (ESP-2), a new platform, ESP-3, was added to the station (stored inside ESP-3 is a new gyro that the spacewalkers used to replace the failed one), disconnecting ESP-3 power cables to prepare it for permanent relocation to the station

Preparing for the relocation of the P6 truss segment on the STS-120 mission, moving two equipment carts from the right to left side of the station and relocating an antenna and upgrade avionics and communication systems. This EVA had to be shortened because of a damage to the left thumb of Mastracchios spacesuit glove. At no time was Mastracchio in any danger, NASA said, but flight rules required Mastracchio to end his spacewalking task as a precaution.

The crew inside used the station's robotic arm to remove a new ammonia tank from shuttle's payload bay and temporarily stow it on the station. Ammonia is used to move excess heat from inside the station to the radiators located outside. The spacewalkers then retrieved a seed experiment outside the Japanese laboratory. Next, the pair installed a grapple bar to the new ammonia tank on the station's truss. The pair also replaced a failed gyroscope that is part of the station's navigation system. They also accomplished several "get-ahead" tasks, removing 11 out of 12 p-clamps slated for a future spacewalk.

Using the station's arm, the astronauts removed the empty ammonia tank from the station's truss and temporarily stowed it on an equipment cart. The new tank then was installed and connected to the truss for use. Connecting the nitrogen and ammonia fluid lines was delayed for this spacewalk because of the delays getting the tank installed. The station's arm then temporarily stowed the old tank on another part of the station's structure until the mission's third spacewalk.

First they finished the complicated change out of the large ammonia tank assembly. Using the station's arm, the crew moved the old tank into the shuttle's payload bay for return to Earth. The spacewalkers also removed the grapple bar from the old ammonia tank and stowed it on a spare parts platform. Finally Anderson and Mastracchio retrieved debris shields from the Quest airlock to return to Earth.

The EVA mainly focused on disconnecting the failed loop A pump module from its location on the Starboard 1 (S1) Truss - including the infamously difficult ammonia disconnections.

They removed a spare pump module from the ESP-3 and installed it on the Starboard 1 (S1) Truss.

The astronauts replaced a backup multiplexer-demultiplexer (MDM) that failed during routine testing April 11, 2014. The box is one of the station's two external MDMs that provide commands to some of the space station's systems, including the external cooling system, solar alpha rotary joints (SARJ) and mobile transporter rail car.