4XX - 4XXX Brush motor MOSFET
If your brush motor will run when connected to a Roomba battery or 9 volt battery but won't run when connected to the Roomba main PCA
then it may be due to a U11 circuit device failure. On most of the circuit boards used today there is an additional location next to the U11 circuit that you can install a heftier MOSFET to run
that brush motor
You can order a replacement MOSFET by sending a paypal payment of $8.25 plus $1.75 USPS to: e_vickery(at)bellsouth(dot)net and mention you want the U11 replacement MOSFET.