C9800 again, ready to retire just looking for replacement....

I am getting a Fatal Error 141, but drum moves up and down no problem. Assume sensor is dirty or failed. I have old parts c9600 can swap, but with all the documentation i have nothing shows the sensor?
is it a flag or a optical?

I assume there is one for each drum, with exception of the K drum (as it doesn't move)....

This happened after running a few hundred B&W envelopes, which we never tend to do...


any OKI tech want to help me directly PM...