Replacing a failed power controller on the station's truss, jettison a refrigerator-sized ammonia reservoir tank and clean seals on a docking port on the Unity module

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.

Installing support equipment on the S1 truss that will allow for an addition to the inspection boom during the STS-123 mission, and installing a system to provide better wireless video coverage during future missions

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.