Faulty Thermister/temperature readings

Hi there, I own a solidoodle press and just recently  I was uncloggling a jam and suddenly my exturder temperture spiked to 350 and then stayed in between 349 and  352.  Realizing this I assumed the thermister was broken so I disconnected it and the temp. reading didn't change. 
It appears to be a board issue because when the bed thermistor is disconnected it goes straight to 0.
I even have a couple extras thermistors around that I hooked up to it and the reading remained the same.
Any ideas of how I should proceed?
Please help,  I do have some technical expertise