I am trying to find out how to check the heatercore door to see if it is stuck. How do I do this? I have tried to backflush the heatercore, didn't work, the thermastat is good so if there is anything else that I should try before I have to replace the core, please tell me. Thanks for all of your help. Also if it helps, the top hose is barely warm, the bottom one is hot as can be.