If Discovery lifts off as planned, it will be the first space shuttle flight since the Columbia disaster in 2003.
“Our flight teams are ready, our flight crew is ready for a successful mission and safe return home,” said Pete Nickolenko, NASA’s test director.
“Countdown continues to proceed very smoothly, with no significant issues or problem,” he said at the Kennedy Space Centre near Cape Canaveral in Florida.
Discovery is due to blast off at 10.39am local time on Tuesday (0039 AEST Wednesday).
Managers at the National Aeronautics and Space Administration said they would go ahead with the launch even if a fuel gauge malfunctions again, on the proviso they can identify the problem.
A planned launch on July 13 was cancelled at the last minute after one of four hydrogen-level sensors in the external tank malfunctioned.
NASA engineers have conducted 161 tests on the system, but are yet to find the root of the problem.
They will run more tests as the huge tank is filled during the night.
Failure of the sensors can result in premature shutdown of the orbiter’s engines during the shuttle’s ascent.
Only two sensors are needed, with the other two serving as backup, but launching the shuttle with only three functioning sensors would mark a departure from current safety regulations.
NASA remains concerned that bad weather could cause another postponement of the “return to flight” that will take a crew of seven, including a Japanese national, on a 12-day space mission.
Should the launch be called off on Wednesday, weather conditions are expected to improve on Thursday.
NASA has until the end of the month to send Discovery into orbit so it can hook up with the International Space Station.
After that, the next window for a daytime launch will be in September.