NASA officials found a technical glitch on the space shuttle Discovery hours before its scheduled launch on Saturday, but it was unclear whether it would delay lift-off, officials said.
"We are still trying to determine what, if any, impact this could have on the launch," NASA launch commentator Bruce Buckingham said at the Kennedy Space Center in Cape Canaveral, Florida.
Buckingham said officials were "very optimistic" they would find a plan to manage the problem.
NASA found that a thermostat controlling the heater of a thrusters at the tail of the shuttle had failed, agency spokesman Kyle Herring told reporters.
The thrusters in question is only used in orbit to make subtle manoeuvres, Herring said.
Preparations for the 3:48 pm (0118 IST, Sunday) launch have otherwise continued as scheduled.
Crews filled the external fuel tank with liquid oxygen and liquid hydrogen, and the seven astronauts made final preparations before entering the shuttle.
The weather also improved overnight, with a 60 percent chance that it would be favourable for lift-off.
If bad weather prevents the flight, the launch would be rescheduled for Sunday.
It would be just the second flight since the 2003 Columbia tragedy.