The toilet is in the astronauts' Russian-built living quarters
International Space Station astronauts are eagerly awaiting the arrival of shuttle Discovery - it is bringing a new pump to mend their broken toilet.
The station's urine collection unit, as opposed to its solid waste unit, has been malfunctioning for several days.
Nasa said it thought a separator pump was at fault, and the three male crew members were operating it manually.
To make room for the new part, Nasa has had to remove other equipment from the shuttle, which launches on Saturday.
"Clearly, having a working toilet is a priority for us," shuttle payload manager Scott Higginbotham said.
The Discovery mission is the second of three to take up key components of the Japanese-built Kibo laboratory.
Space urinals generally use jets of air to guide waste down a tube into a container, where it is then separated into liquid and gas.
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On its website, Nasa said the crew first realised something was wrong when they "heard a loud noise and the fan stopped working".
The crew replaced many of the toilet's working parts, but had to adopt "temporary manual operation of the pump", and a backup system for the separator unit.
Nasa said one of its employees was rushing from Russia to Florida with the spare parts for the Russian-built toilet ahead of the shuttle launch.
A 50cm-long (20in) pump and additional hardware, weighing about 16kg (35lbs), would be carried as hand luggage on a commercial plane, Nasa said.
The employee is expected to drive the parts to the Kennedy Space Center, and they should be packed on the shuttle on Thursday.
For a while, the crew was told to use toilet facilities in the Soyuz capsule docked at the ISS, and several other backup solutions are available.
The space station's solid waste unit is said to be functioning well.