Fixed an old Roomba today with this tutorial. The Roomba was going forward a bit then hitting an invisible wall. Then it would back up and spin.
There is a camera under the front wheel. The wheel is half white and half black.
We took out the front wheel, shined up the white part, and cleaned the camera under the wheel.
It worked right away after that.
iRobot need something like an Apple Store but a garage for cleaning/maintenance.