One of the cooling systems was shut down because of fluctuating temperatures, leaving the space station with just half of its cooling ability on Wednesday.
NASA spokesman Bob Jacobs said there was no immediate risk to the six crewmen on board. But some non-critical equipment of the massive orbital outpost were powered down.
Engineers suspect a valve inside the pump was faulty and ground controllers moved electrical power supplies to the other cooling loop, Jacobs said. These loops circulate ammonia outside the station to keep equipment inside and outside cool.
Mission control is trying to figure out if the problem is due to a software glitch or hardware before deciding on whether an emergency spacewalk is needed in order to diagnose and fix the issue. Officials say the crew went to bed as normal, while engineers on the ground tried to troubleshoot the problem.
In the unlikely event that the problem cannot be fixed, the space station may have to be evacuated.
The crew on the space station is made up of American astronauts Rick Mastracchio and Michael Hopkins, Russian cosmonauts Oleg Kotov, Mikhail Tyurin and Sergey Ryazanaskiy and Japanese astronaut Koichi Wakata. The most recent occupants arrived in November and are supposed to stay until March of next year.
NASA hasn't commissioned any spacewalks since one was aborted this past summer. That's when an Italian astronaut nearly drowned when water started filling his helmet. Engineers still don't know how that happened.
ABC News and The Associated Press contributed to this report.