One Plus One Touchscreen

Hey guys, I want to buy a "defective" OPO for 100$. The sellers told me that a part of the touchscreen (about 13%) is not working. He says it works every once in a while but it is really rare. I was wondering if I would be able to fix this trough a Kernel or a ROM update (he runs CM12S) or if I need to change the screen completely. The guy says he had the phone for 2 months and he hasn't try to fix it because he has an iPhone (LOL)

Thanks for your answers!

Chris