From SpaceX on Facebook: “Engineers replacing failed valve on engine #5 following today’s abort. Data review Sunday, if all looks good next attempt is Tues. May 22 at 3:44 AM ET”.
SpaceX on Twitter notes a “…faulty check valve…”.
Elon on Twitter: “Engine pressure anomaly traced to turbopump valve. Replacing on engine 5 and verifying no common mode.”