If you have a v1 model and have not fixed the cable chain heater wire issue then this may be your problem. This is the message from Wanhao:
We have one design defect which may cause the cable fire hazard. We request all Wanhao i3 customer check and correct the following point to avoid any possible hazard.DESIGN DEFECT: All the WANHAO i3 shipped before 27th Aug 2015 has the crimp(connection point between stainless steel cable and brass cable） inside the drag Chain. The drag Chain keeps moving can cause the crimp fatigue and break(check pic below). Then it can cause heating failture or circuit short.
To drag the Heating tube crimp out side the drag chain. So no matter how much moment the drag chain move. The crimp shall never wear and break.