Just recently, our Ultimaker clone 3D printer, theExtrabot 3020 E3D, had a problem with its heater cartridge. We've been running it pretty hard for almost a year without any major problems so we've been lucky that this was the first real problem we faced.
We first attempted to find and solder the faulty point in the line, but it turns out we never really found where the fault was. Eventually we had to give up and replace it. Because many 3D printers are opensource including ours it was very easy to find the exact heater cartridge we needed to replace it. We got it in about a day through the mail, and the same day it was delivered we installed it and were printing again.