I am unable to check codes on my 05 C6. I have a CAN capable scan tool, I have also used other peoples scan tools. I even went to a local shop and they where unable to check codes. Everytime I get the same message on the scanner; unable to communicate, ensure vehicle is on. I have gone to the local dealership twice and both times they where able to check codes. After leaving the dealership, I was able to connect with my scanner. Then a few days later I was unable to check codes again. What could be causing this?