Roomba with stuck bumper sensor
The above video shows a roomba with a stuck front bumper sensor. As you can see, the robot keeps backing up and trying to turn away from
the phantom obstacle it thinks is in front of it. This particular sensor was triggering because lint had gotten into the optical beam path and was blocking light to the photodiode detector even when the bumper was not being hit. I was able to repair it by removing the bumper cover (unscrewing 4 screws) and vacuuming out the hole where the plastic pole from the bumper goes between the IR diode and photo-transistor.
I found the following website to be very useful when debugging the sensors on my Roomba to determine which of the front bump detectors were malfunctioning:
Update: Link dead, wayback machine version:
Summary: Hold the spot and clean buttons down. Push the power switch. Keep holding spot and clean buttons down while the robot gives five ascending beeps, and five descending beeps. Release spot/clean and you are in diagnostic mode.
Test zero: Flash all leds
Press clean to advance to the next test.
Test one: Bumper Sensors
Test-Two: OUTER, CLIFF-SENSORS PAIR
Test-Three: INNER, CLIFF-SENSORS PAIR
Test-Four: WHEEL-DROP SWITCHES (3 ea) and WALL-SENSOR (1 ea)
Many other tests follow, but deal with motors, etc and can be dangerous if you let the robot run itself into something, because it will NOT automatically stop moving. Read the above linked page if you want to do any of the other tests.