For a week leading up the much-anticipated launch of Boeing’s Starliner test flight on Tuesday, officials with NASA and the aerospace company said the spacecraft and its Atlas V rocket were ready to go. Their big concern, they said, was weather—with frequent afternoon thunderstorms along Florida’s east coast.

On Tuesday morning, a few hours before the launch window opened, weather conditions along the coast looked pretty good. But Boeing had to scrub the launch attempt anyway, citing a problem with a valve in the spacecraft’s reaction control system, which helps the vehicle maneuver in space.

Boeing and NASA engineers then spent Tuesday afternoon attempting to determine the cause of “unexpected valve position indications” within the spacecraft’s propulsion system. Presumably, they were trying to determine whether these valves were actually in the wrong position in reality, or if the indications were due to some sort of errant sensor reading.