This is probably not your issue, but i had something similar happen. turns out i had my pinout from my sanguino incorrect. the pin wasn't an output for anything, so it was just floating and told my PWM board to turn it on.
however, if your thermistor is still detecting 17C when the temp is obviously higher, then that probably isn't your issue. try setting the temp to something under 17, if the heater shuts off, then your thermistor/board/heater tables are messed up.
andres