Flashing yellow O2 indicator.

Service technician here (lots of experience with concentrators). I am baffled at what would cause this concentrator to flash continuously. PSI between 16-29 while in operation, O2 output at 5LPM is 94% No leaks, changed out the sieve beds, rebuilt the compressor, replaced capacitor, mixing tank, tubing, main PCB and O2 sensor boards, power switch, 9V, new valve kit for all 4 valves, PE valve, exhaust muffler, Teflon taped all connections and leak checked. Temperature is well within operational parameters. I am at a loss at what would cause the low O2 indicator light to flash constantly. Thanks in advance!