B737 Undifferentiated or Other Model
Flight Crew Commercial; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Troubleshooting
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Weight And Balance
Flight Crew Diverted
Aircraft; Company Policy; Procedure
We departed ZZZ with MEL 38-XX. Per the MEL; lavatories were tested for proper function above 16;000 feet and again at level off. All attempts failed to produce the expected results. Flight attendants attempted to operate all of the lavatories numerous times; yet they all remained inoperative. We contacted Dispatch and began a turn back to ZZZ.[Company Operations] determined ZZZ1 was a better choice to accommodate passengers for a follow on flight. After obtaining landing data and conferring with Dispatch; we elected to land overweight. ZZZ1 conducted a runway change. We obtained new landing data and completed an uneventful approach and landing. Uneventful overweight landing conducted on Runway XXR in ZZZ1 after consultation with Dispatch and [Company Operations] and obtaining landing data.Placing MEL 38-XX on an ETOPS flight might not be the best choice. Checking the system occurs above 16;000 feet and if not successful; requires either an overweight landing or a very lengthy delay to burn off fuel and reduce weight.
B737 Captain reported all of the aircraft lavatories did not work so they were forced to divert and land overweight.