Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine; Flight Crew Instrument
Human-Machine Interface; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Provided Assistance; Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance
Aircraft; Human Factors; Procedure
First Officer auto uplinked flight planned route via ACARS.  Per standard procedure before beginning the 'Before Start Checklist' the First Officer and I pulled up our PDC clearance and observed it was exactly as requested.  We then made sure the FMC had the same routing as our clearance.  I read the routing directly from the FMC screen and the FO confirmed it exactly matched the clearance and our flight plan.  I then read the leg waypoints in order from the FMC while the FO checked them against the flight plan.  All checked properly.  This is standard procedure. The clearance was: SNA STREL4 TRM J169 TFD J50 ......Normal flight to TRM.  Unbeknownst to me the Nav system had somehow dropped out J169 and we did not recognize this. Therefore after TRM the aircraft turned and proceeded directly to TFD.  LA center came up and told us to turn 15 degrees left to avoid military airspace.  He then asked us what our routing was.  I confirmed the original routing J169 and admitted I was baffled as how our flying directly to TFD occurred.In reprising the event I still don't understand how or why J169 dropped out of the FMC.  Both the FO and I are certain we remember confirming J169 connected TRM to TFD and at no time during the flight did we make any route modifications.I pride myself on being vigilant and following standard procedure. I will redouble my efforts to confirm the connections from the SID and STAR to the Jet routes.
B737 Captain reported a course deviation after the FMC lost a waypoint in cruise.