Engineers have raced to isolate the glitch, and have come up with two likely candidates.

The current launch window is open until 31 July, but may be widened into the first week of August.

Discovery's mission 12-mission to the International Space Station marks Nasa's first shuttle launch since the loss of Columbia on 1 February 2003.

Wiring switch

Dr Mike Griffin, the US space agency's administrator, said he hoped the problem did recur during countdown tests, so engineers could resolve the problem once and for all.

"What you want of Nasa is that we make the right technical decisions, that we do the right thing, to the extent that we can figure that out, which is hard," he explained.

"We can't restrict the range of our options to those things that are going to present well."

The launch attempt on 13 July was scrubbed when one of four identical engine cut-off (Eco) sensors failed a routine countdown check. The sensors act as fuel gauges, monitoring the volume of cryogenic hydrogen in the shuttle's external tank.

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For Tuesday, the wiring between sensors number two - which played up last time - and number four has now been switched, to try to isolate the glitch.

If engineers see a failure in either of these sensors, managers say they will understand the problem well enough to fly Discovery.

However, if an unforeseen problem with the other sensors arises, they will have to abort the launch.

A 'huge effort'

Engineers have so far identified and fixed three electrical grounding issues in the sensor wiring system and also suspect electromagnetic interference from other hardware could be the root cause of the problem.

Mr Hale brandished an example of the sensor involved at a news conference here at Kennedy Space Center.

Though engineers have completed a battery of "ambient" tests on the sensor system, they have to wait until the early hours of Tuesday to check how they perform with a full tank of cryogenic liquid hydrogen.

Mr Hale countered suggestions that Nasa carry out a "tanking" test, in which the external tank is filled up with cryogenic propellants, before attempting launch.

"I think we're all still struggling a bit with the ghosts of Columbia, so we want to make sure we do this right," said Mr Hale.

"Are we taking care enough to do it right? Based on the last 10 days' worth of effort, the huge number of people and the tremendous number of hours that have been spent testing and analysing - I think we're coming to the right place."

At the right time

The sensors at the heart of Nasa's troubleshooting operation ensure the orbiter's three main engines shut down before its fuel runs out, avoiding the potentially catastrophic scenario of the motors running with empty tanks.

But the agency also needs to be sure that the sensor glitch does not lead to the alternative and equally unwelcome scenario in which the engines shut down too early because the system believes wrongly all the tanks are empty.

An engine shutdown before Discovery reached its intended orbit could force the crew to abort the mission and make an emergency landing.

The US space agency has set Tuesday's launch for 1039 EDT (1439 BMT; 1539 BST).

Discovery's 12-day mission will deliver parts and supplies to the ISS. It will also give the astronauts a chance to test new safety features on the shuttle brought in following the loss of Columbia.

If Tuesday's launch goes ahead as planned, the shuttle will return to Earth on the morning of 7 August, landing at Kennedy Space Center.