Phaser 740 error 12 (solved) 1

[alfasw33]

Hi folks,
after replacing the imaging unit in the Phaser 740, the printer came up with error 12 (laser). The recommended restart did not help, it had also nothing to do with actual very low ambient humidity (which should be above 20) and we had 15 (and I started wondering if that could be the reason...)
Inserting the old imaging unit made the error disappear...
Xerox support suggested to make NVRAM reset, but while talking with them I found another hint in the internet, concerning error 30. And this solved error 12 too :
So just remove the imaging unit (cover it!) and clean the sensor, which is the small transparent plastic part, located inside the printer on the rear panel. See this for details:

http://www.officeprinting.xerox.com/userdoc/P740/740ADV.pdf

on page 110.
Hope this is helpfull for someone...
Peter