You can generally fix them yourself - they are very simple. They consist of a small incandescent bulb and a photocell with a pigtail ( I know, I know, it's a stainless steel squashed-tube bourdon device ) pressure arm between them.
Without disassembling anything, in a darkened room, from the front of the pump, you can look at the top gap between the sensor SS block and the front panel. You should see a small amount of incandescent light ( but you won't see the actual bulb ).
If it's totally dark, it's likely the bulb has gone. From memory, it's the same as used in the Waters RI-401 detector ( Waters 40722 ), but a supplier, such as Radio Spares, should have similar beasties, usually called " wire ended axial filament indicator lamps". They usually are larger, but the bulb doesn't have to be that profile though, as long as the bulb fits and is of similar voltage and illumination.
My intention is to replace the lamps with a resistor and white LED when they fail in my old Waters RI and 6000A systems.
If it's a calibration issue, you can withdraw the pigtail assembly and twist it slightly to make it work.
The electronics are all discrete and, from distant memory there are adjustments on the sensor circuit ( as well as the front Zero adjustment ) but you don't need to touch them usually.
