Something is impeding the drive train. Usually, it is the brush loaded with stuff it picked up. If the brush is clean, it obviously is not a clogged brush. The next step requires you to disassemble Roomba. Take brush, roller, and brush keeper. Easy part. Remove the bottom plate. It is held on with screws. Don’t lose them. You also mustunscrew and remove the circular sweeper thing. The bottom should come off. Now you must remove the entire piece that holds the brushes. Careful because the motor wires are attached. There is enough slack that you need not - and should not- unwire the motor. The piece that holds the brushes comes out. But not easily. There is a spring plastic catch right and left. You must depress both simultaneously (takes two hands), grip the brush holder piece (third hand) and hold the body of roomba attune rear (fourth hand), and pull the assembly out (fifth hand). People with five hands find this easy. It is almost impossible with only two hands. The entire green thing comes out....

I have completely disassembled mt Roomba 500. Cleaned brushes meticulously, disassembled the drive train. Cleaned gears and axles. Still get Error 2! I infer that the resistance is upstream of brushes and gears. Perhaps the motor shaft itself. That will be a very tough repair for me