SC400 on Ricoh MPC2051/ 2551 is really a pain is ass. Service manual says that it is ID sensor (Image density sensor) adjustment error. Here is procedure to eleminate this error.
- Remove transfer belt unit from the machine.
- Now you will be able to see ID sensors, exact location of the sensors is just beneath the transfer belt unit.
- Clean the ID sensor with a wet cloth, Do not use dry cloth or tissue papers.
- There is a metal plate on front side of the transfer unit, remove it (2 screws) and clean it well.
- Refix all removed parts.
- Switch on the machine.
Here you go, SC400 should have vanished by now.