45-20 is color registration failure. To do this, it creates and developes 7's and backward 7's on both far sides of the transfer belt. It then uses the sensors that are under the shutters to read the timing and angularity of both sides for each color. If the belt is off track, it will mis-read and code. If the sensors are covered, it wil not read and code. If the belt does not turn right, same thing. If you have something that keeps one of the colors from deleloping be it a bad corona, development unit, drum, to whatever, it will code. Have your tech go into 36 mode and run each color one at a time with full laser power and others off to make sure each color is full across the page. If that is ok, and you have already tried the belt unit and the sensor unit, it will narrow down pretty fast. I don't know what else to tell you.
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