The countdown cleared away doubts about the fuel sensors whose erratic behavior had forced a two-week delay in the flight.

They vehicles come equipped with a fuel sensor that automatically adjust the air/fuel ratio to optimize running on a particular fuel.

One of the flight computers had to be replaced along with nine fuel sensors and a critical electronic unit or "black box".

Vass tapped into the car's computer network and established a new line of communication with the Prius's fuel sensor.

There was also speculation during the flight that fuel sensors had malfunctioned, providing false information on fuel quantities.

Then the shuttle will be rolled back to its assembly building for a new fuel-line heater and to replace malfunctioning fuel sensors, Mr. Parsons said.

In countries with high prices on gasoline external fuel sensors are used to prevent cases of fuel theft.

The Discovery's flight last July, the first since the Columbia disaster, was plagued by a problem with the fuel sensors.

The system uses an angle sensor and a number of fuel sensors to provide a maintenance-free ignition system.

This was later found to have been due to greater propellant 'slosh' than expected, uncovering a fuel sensor.