An item which is broken, defective or substandard. Two explainations have been historically applied. The first is that in olden times when food was scarce, people would leave the bones, fat and undesireable portions behind after eating their meal. These second-rate items would be used for soup the next day, so as such, the poor-quality leftovers would "go to pot". The second (and more plausible) explaination is that in the days of the industrial revolution and early mass-production, assembly workers would occasionally find a defective or out-of-tolerance part which was not suitable for use. This part would be sent back to the smelting room to be melted down and re-cast a second time. Since the smelting was done in a giant pot, these defective parts had "gone to pot". In either case, the phrase gained popular use by the American homeowner who would occasionally wear out an item which would fail- often at an inconvenient time.
Frank didn't want to take his car in for routine maintenance. I wasn't surprised when his vacation was ruined after the car went to pot last summer.