Astronauts aboard the International Space Station spent much of Sunday working on a urine recycling system designed to turn waste into drinkable water, dpa reported.
The system has a problem with a centrifuge on a distillation component of the urine processor assembly and would shut down during tests before it could complete its task.
On Sunday, astronauts removed grommets from the unit that NASA believed were hitting a sensor that caused it to shut down after about two hours.
The work appeared to briefly fix the problem, but it shut down again after three and a half hours, ISS commander Mike Fincke said. The unit was able to process 3.8 litres of urine, or about a third of a tank, before shutting down, he told ground control.
NASA scientists on the ground were to continue troubleshooting and hoped to get the system up and running before the space shuttle Endeavour is due to leave the ISS on Thursday. The US space agency has left open the possibility of extending the mission by one day to deal with the problem, but no decisions have been made.
Endeavour astronauts will conduct a fourth spacewalk on Monday to work on joints on the solar panels that power the ISS.