Hi, I had the same problem error 31 every time I activated the Roomba, it was driving me mad I phoned the help line but to be honest they were useless telling me that a reboot will sort it. No amount of rebooting will sort this it is a problem with the sensors. I bought a new set from USA via eBay about $35 replaced all 6 and had no problem since not one error 31.
It’s fairly easy to do just watch the YouTube video on how to remove bumper from Roomba and replace each sensor one at a time just make sure that the cable length and end plug match when removing and replacing. There is a slight difference on some of the end plugs, just take your time, it’s quite easy.
I have not had any more error 31 since, by all means try cleaning them with a soft cloth first, DONT USE ANY CLEANING SOLUTION.