500 series (pet version from Costco)
Whenever a heavy-ishload (25-30 lbs) is put on it, it does not want to carry it in a straight line. It will start turning every which way as if it's trying to dislodge itself, and after 10-15 seconds, it will stop and red lights will flash. I assume it also says something, but I'm deaf so I have no idea what it's saying.
At first I thought that the main wheels were bottoming out and causing drag, or at least setting off a sensor that told the Roomba was stuck, or getting stuck.
So I removed the wheels, cut out some heavy duty foam inserts and put them between the wheel and the fender walls to keep the suspension from compressing. Big mistake as it sat several inches too high and would not even go. I then realized that when it was the right side up without the foam inserts, the wheels were indeed "bottomed out" therefore it's not the wheels that needed to be raised, or held open. Must be a sensor or something else completely that needs to be messed with to get it to carry a load without freaking out. I removed the inserts and it's rolling around fine now, but still won't carry a load.