The Happy Valley amusement park has been ordered to keep one of its roller coasters shut down until it can determine the root cause of a breakdown that left 16 riders stranded earlier this month, a quality control official said at a press conference Monday.

The Shanghai Municipal Bureau of Quality and Technical Supervision has also ordered the Songjiang district amusement park to thoroughly inspect all of its roller coasters in response to the recent malfunction on its Mega-Lite coaster, said Shen Weimin, the bureau's vice director.

The Mega-Lite ground to a halt mid-ride on October 19, leaving its riders stuck about 25 meters above the ground for about half an hour. The malfunction forced the riders to exit the roller coaster via its emergency stairs.

The park pinned the malfunction on one of the roller coaster's 200 sensors, which it said went off in error. The sensor serves as a fail-safe device that automatically shuts down the roller coaster if it senses a problem.

The park's management told local media that they couldn't classify the October 19 shutdown as a problem with the roller coaster itself. Instead, they said the sensor showed a false positive, which sometimes happens when it gets too warm outside.

It wasn't the first time that Happy Valley's management has blamed a breakdown on a faulty sensor.

On September 28, 28 riders on the Diving Coaster ended up stuck 60 meters in the air after one of its sensors went off, the Xinmin Evening News reported. The park's management said the malfunction occurred because the sensor had been set at too sensitive a level.

The bureau is planning to create standards that specify how frequently roller coaster sensors have to be replaced, Shen said. Following a recent bureau inspection, Happy Valley swapped out 32 sensors on its rides.

Happy Valley does not have a specific timetable for when it replaces its sensors, said Xu Fei, the park's press officer.

"The engineers and technicians decide whether to replace a sensor based on their own observations and experience during routine inspections," Xu told the Global Times.

Happy Valley has not operated the Mega-Lite since October 20, though the Diving Coaster remains open to the public.