Some customers reported constant failure of the Hot-end Peek part in the FELIX 3.1(and older) hot-ends. The causes for this are:

• Thermistor connected to the wrong hot-end in case of dual head.
• The thermistor is defect, it measures a lower temperature than it is in reality. The peek part melts when the temperature is above 350 degC. The increase in temperature should be around 3 degC per second. Also a good indication with dual head printing, is to turn on both heaters at the same time from a cold state (less than 30degC). By the time one of the two hot-ends reach their setpoint temperature (a good temp would be 180 degC), the difference in temperature should be less than 10degC.
• The thermistor fell out of the heater body of the hot-end.Only an issue with older firmare (pre v0.92)
• Mainboard thermistor circuit is defect or not functioning right. Control board replacement can solve this issue.

A recommended fix is to install a full metal hot-end from the TEC 4. 

Note: Repetier firmware from version 0.92 and upwards have a safety feature built in which prevent the ability of overheating. Always make sure to use the latest firmware on your printer. Get it from here: www.felixprinters.com/downloads