Thank you Blackcat for your assistance and info.|
i've checked and found that if i try to scan when the top of the scanner (with the glass) if off, i get this error code. in this case the lamp is under the calibration white strip, so when there is no reflection this error is presented.
i saw that the optics are dirty and cleaning it solved the problem.
I talked to a service manager of a large firm in Israel, and he told me that cleaning the optics is what they usualy do to solve it, and in addition, they type in the service mode code U078 to reset it.
I don't have the service manual, so I don't know what it code 078.