Not honoring warranty claims

I have a HP laptop (HP is also useless in my opinion) that is to be repaired under 'factory warranty'. I took it to my local IT center to fault find why the GPU stopped working. They found that there was a short on the motherboard. Sent all the details to these useless people, now they recon the warranty is void. BUT HOW WHERE THEY SUPPOSED TO DETERMINE WHAT WAS WRONG?