As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CABIN CREW REPORTS: NONE OF THE TOILETS WILL FLUSH IN FLIGHT AT CRUISE ALTITUDE. GROUNDED - YES. DIVERSION TIME 56 MINUTES - ACCOMPLISHED B737-900 FIM TASK 38-30 TASK 821, FOUND THE WASTE DRAIN VALVE CONTROL HANDLE OUT, PUSHED HANDLE IN ALL THE WAY, ALL LAV TOILETS OPS CHECK GOOD.