Does it already start raising before you turn on heater?
If yes then there might be hardware defect setting the output to high all the time, e.g. a solder bridge from 5v to pin.
As you read the temperature there should nothing to prevent control only wrong heater pin (then it should not start at all) or the hardware defect or you have a inverting output, but I only know one board having had this for a short while as it is a safety feature to need to turn it on explicitly.
