B737-800
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Situational Awareness; Communication Breakdown; Confusion
Aircraft Equipment Problem Critical
General None Reported / Taken
Human Factors; Aircraft
During preflight for our first two legs both of us noticed the crew O2 was pegged past full which; in retrospect; looked out of the ordinary but neither of us mentioned it to the other. We ended up diverting to Ontario on our last leg due to weather at our destination. On the post flight we noticed the crew O2 was now pegged in the other direction; low; and called maintenance. The mechanic said there was a loose connection; possibly the cannon plug; for the cockpit indicator and that the actual bottle pressure was 300 PSI which was well below the 1;000 PSI minimum. To me; it appeared the airplane had flown for some period of time with significantly low flight crew O2 that was unknown to the pilots due to a faulty gauge. Had a situation occurred where we needed the oxygen it might have been a problem. I suggest putting some information out to line pilots about what a faulty crew O2 indication might look like; i.e. fully pegged past full and how often the crew bottle pressure is checked by maintenance. This may just have been a random event but if it happened once it could happen again.
A B737-800 flight crew flew two legs before discovering their flight crew oxygen bottle pressure well below minimum upon completion of their final leg. The pressure gauge had been reading abnormally high which did not trigger further assessment earlier.