VALLEJO, Calif. (AP) — A roller coaster that stalled at a Northern California theme park, stranding a dozen riders 150 feet above the ground, is back in service.

Superman Ultimate Flight at Six Flags Discovery Kingdom in Vallejo began running again over the weekend.

Park spokeswoman Nancy Chan says engineers determined the July 29 stall was caused by insufficient power to the coaster's launch system. The problem has been corrected.

No one was injured during the incident. The riders were stuck upright for about 90 minutes before the roller coaster was restarted and came down, traveling backwards.