CAPE CANAVERAL, Fla. (AP) _ Problems that nearly aborted the flight of the space shuttle Challenger were caused by three brittle, broken thermal sensors sending out false signals of overheating fuel pumps, an official says.

The belief that the sensors might be faulty had prompted flight director Cleon Lacefield to overrule the readings and order the mission to continue.

NASA said after the flight ended Aug. 8 that it believed bad sensors were to blame for the shutdown of one engine and near shutdown of another.

All three sensors removed after the shuttle landed were defective, Dominic Sanchini, executive vice president for production at the Rocketdyne Division of Rockwell International, said Monday. Rocketdyne is the prime contractor on the shuttle main engines. ''We found exactly what we expected,'' he said.

Sanchini said the basic problem involved the way the wire elements in the sensors and the tubes in which they operate are twisted and bent during manufacture before they are subjected to 1,700-degree heat to make them uniform.

''Under certain conditions, we found that the wire can become brittle and break,'' he said. ''Under this circumstance it can no longer properly measure engine temperatures. It's what we call an electrical open; it's like a circuit breaker opening.''

Sanchini said a new production process has greatly improved subsequent sensors and that Rocketdyne has recommended the new ones fly on Discovery when it is launched Aug. 24. He said the space agency is completing its own testing of the improved devices.

Challenger was 5 minutes, 45 seconds into its flight on July 29, when one of its three main engines, No. 1, suddenly quit, nearly three minutes before it was supposed to do so.

The engine was ordered shut down by a computer when one of the defective sensors, then a second, gave a reading that a fuel pump was too hot and could cause damage.

The computer also commanded the other two engines to burn 86 seconds longer than intended, providing enough power to propel Challenger and its seven-man crew into orbit 195 miles up. That was 46 miles lower than planned, but the crew was able to complete a weeklong mission and stay up an extra day for science work.

While the remaining engines were firing, another sensor gave indications that the fuel pump in engine No. 3 was overheating.

But other radioed readings showed the pump was operating normally, and Lacefield, believing the sensor was faulty, ordered it disabled before the computer could shut down another engine.

He said later that had the second engine been lost, Challenger would have fallen back to Earth over the Mediterranean Sea. In that event, he said, the crew would have attempted an emergency landing on the island of Crete, but ''more likely we would have been in the water.''

Sanchini said the new production process involves a more careful control of temperatures at which the devices are heated. He said 10 sensors produced under the procedure have been subjected to engine firing tests and have recorded 15,000 seconds of operation with no problem.

There are four sensors on each shuttle engine, two on the fuel pump and two on the oxidizer pump.

Sanchini said 216 have flown on shuttle missions and only six have failed. Challenger's mission was the first on which more than one failed. It takes the failure of both sensors on a pump to send a false signal that would shut an engine down.