bulb replacement FLD G1321A

I guess the error is EE 6721 - no light at reference diode despite lamp is on. This is certainly not caused by a lost calibration. It points more to a severe problem with the Xe-light. You will need an engineer onsite to check for a contacts problem of the wires from the FLL board to the FLF board. Also, the FLL itself could be faulty. Less likely is a defective FLF or FLM board, a bad lamp or a faulty safety switch.

At one point, the lamp turned off. Have been replaced.

That is, the failure of some kind of power switch in its trigger, or power?

At one point, the lamp turned off. Have been replaced.

That is, the failure of some kind of power switch in its trigger, or power?

An FLD is different to a UV detector. On those, people tend to replace the lamp first, if there is a problem, but on FLDs the lamp is less often the source of problems. Of course, also the Xe-lamp has a certain life-time, but it's way more of what we expect for a D2-lamp. In your case I would highly suspect the trigger-pack (=FLL). A trained engineer is required to replace it, check for proper alignment and also exclude all the other possibilities, I mentioned. Once the instrument is working again, an intensity test needs to be carried out and checked, plus the FLD needs to be calibrated with the Glycogen solution.