An update from SpaceX:
“Today’s launch was aborted when the flight computer detected slightly high pressure in the engine 5 combustion chamber. We have discovered root cause and repairs are underway.
“During rigorous inspections of the engine, SpaceX engineers discovered a faulty check valve on the Merlin engine. We are now in the process of replacing the failed valve. Those repairs should be complete tonight. We will continue to review data on Sunday. If things look good, we will be ready to attempt to launch on Tuesday, May 22nd at 3:44 AM Eastern.”