Discovery 2 td5 gear box problem

Hi I am new to the group and looking for some help if possible. Our discovery’s gear box failed so we got a second hand one from eBay. Our friend has fitted it but comes up with a fault. We have been told to find someone with a diagnostic machine to erase the fault but can’t find anyone local that does an automatic. I was wondering if anyone knows anything we can try. Thankyou

Welcome