CAPE CANAVERAL - Erratic readings from one of three fuel cells aboard shuttle Atlantis forced NASA to scrub today's attempt to launch the spacecraft and a crew commanded by Brent Jett Jr. of South Florida.

The space agency hopes to try again Thursday at 12:03 p.m., but it first must diagnose and resolve the malfunction.

The fuel cells generate electricity for all of the shuttle's critical functions during flight and produce drinking water for the crew. Flight rules require three operating cells before a mission can begin.

But engineers for the National Aeronautics and Space Administration said that one of the cells produced a low voltage reading -- and then the other two spiked higher to compensate -- during a prelaunch test overnight.

''We had an indication during fuel-cell start up of a high voltage reading,'' said NASA spokesman Bruce Buckingham. ``It's something we didn't expect to see so we went back to do some troubleshooting.''

As a result, technicians did not begin fueling the shuttle and NASA canceled today's attempt to launch Jett and five other astronauts on an 11-day mission to install an additional solar power module on the International Space Station.

Located under the floor of the shuttle's cargo bay, fuel cells are not easily accessible, and diagnosing and repairing this kind of problem can be difficult.

Engineers will work on it throughout the day; mission managers planned to meet at 1 p.m. EDT to discuss their options.

Repairs to the cells often require removing a shuttle from the launch pad and rolling it back to the hangar, a process that could delay liftoff for weeks.

If Atlantis cannot be launched by Friday, blastoff likely will be postponed until late September or even late October to comply with daylight launch requirements and avoid a conflict with a Russian flight to the station.

Jett, 47, grew up in Fort Lauderdale and attended Northeast High School before joining the Navy and becoming a fighter pilot. His parents still live in the area. This will be his fourth mission in space.