Hi, I know I can't speak Japanese, but I am here to help. Error 6 ( 2:40 ) means the cliff sensors are faulty. In this case, I saw you trying to wipe the cliff sensors surface, but this does not help. So instead, this can be a wiring issue, or even the motherboard issue. I also bought a second hand Roomba 980, which also has the error 6 due to water damage that causes the wiring to rust and disconnect. I solder new wirings and IR LEDs to fix the problem. Also, I think the wheels are having problems from their encoders, which couldn't control the speed of the wheel. replacing the wheels can stop this problem. I hope my advice helps on reviving this Roomba 552. Thank you (arigatou)!