2003j20

happened at 190000KM and ive heard this problem being quite common for this year yet there isn't a for sure theory or fix about why. If the door is stuck then fine but my mixer motor and valve both work as they should and my coolant is clean clean.I just flushed core and cycled for air pockets. Nothing has worked yet. There has to be something we or I'm missing. I don't subscribe to flukes with machines. Machines never lie is my saying. Pls help..... its winter up here...hahaha