Mastracchio and Hopkins spent more than seven hours outside the ISS.
"Power was successfully restored to the ammonia pump module," NASA said. "However, flight controllers will perform more tests before restarting the pump and returning it to full functionality."
Tuesday's excursion was the second Christmas Eve spacewalk in the history of NASA.
One of the ISS' two external cooling pumps shut itself down Dec 11 after sensors detected a temperature anomaly.
While posing no threat to the crew, the pump malfunction forced a shutdown of non-critical systems. NASA hoped to correct the problem through a software fix, but engineers ultimately concluded the unit would have to be replaced.
Mastracchio and Hopkins removed the defective pump during a spacewalk last Saturday.
NASA provided live broadcasts of both spacewalks.