Date: 201902
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
ATC / advisory: Center ZZZ
Aircraft component: Indicating and Warning - Fuel System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Troubleshooting; Confusion
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical; Inflight Event / Encounter Fuel Issue
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Landed As Precaution; General Maintenance Action
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: The cockpit fuel gauges showed the required fuel load for our flight during our pre-departure setup. Approximately 2 hours into the flight; the fuel tanks abruptly showed an imbalance in exceedance of the maximum amount. A cross feed of fuel was made to bring the tanks back into balance. It was about this time that our FMC fuel began to show that we were below the amount required for our point in time of the flight. We notified Line Maintenance of the problem and began doing a series of checks related to fuel burned and noticed nothing unusual. Our total fuel for arrival into ZZZ was beginning to show a steady trend of depletion so that when it reached the point that the FMC was showing us with an arrival fuel of 3700 lbs. (approximately FAR minimums); we decided to land at ZZZ1 which at that time was about 80 NM ahead of us. The gauges on final approach at ZZZ1 were showing a total of 6000 lbs. and balanced but after landing during taxi in they jumped up to a total of 7200 lbs. and showed an out of limits balance again. Upon a thorough maintenance inspection by [Company] mechanics who were brought in from ZZZ overnight; we were told that we actually had 3400 lbs. of fuel on board at shutdown in ZZZ1.Needless to say; had we continued on to ZZZ; it would have probably not been a good ending. The right quantity indicator was deferred inop; fuel was metered on by the truck and we ferried the aircraft to ZZZ the next morning. It is my opinion that the improper fuel was loaded and no cross checks were done to ensure that the gauges were in fact functioning properly.


Second narrative: Captain and I were operating flight to ZZZ and a couple of hours into the flight we had a fuel imbalance. The right tank showed out of balance so we took corrective action; rebalanced from left to right and continued as normal with no other issues. We turned the plane in ZZZ and operated back to ZZZ1. We climbed up to our final cruise altitude of 40;000 and proceeded as normal on our filed flight plan.About an hour into the flight we noticed our Fuel over Destination (FOD) was about 1500 pounds lower than it should be. It was showing landing ZZZ1 with ~4.7. As the flight proceeded the FOD kept dropping. About 2 hours into the flight we got another fuel imbalance on the right tank. We corrected by rebalancing from left to right. All other indications were normal. Fuel flows were evenly matched; the left motor was actually burning about 50 pounds per hour more than the right. After roughly another hour our FOD had dropped to 3.7 over ZZZ1 and it was at this time that we decided neither of us were comfortable with the remaining FOD. We diverted into ZZZ2 and when we touched down the total fuel on board showed 6.0 with an imbalance now in the left tank. We parked and shutdown and our fuel sitting on the ramp now showed a total of 7.0 with ~2.5 in the left and ~4.5 in the right. The next morning talking with the mechanics who had worked on the aircraft that night; they had stuck the tanks and found we only had a total of 3400 pounds of fuel on board the aircraft; even with the gauges showing 7.0. 3400 pounds would not have been enough fuel to complete the flight to ZZZ1.


Synopsis: B737 flight crew reported inaccurate fuel indication led to a diversion.