Diagnosis of C-cables
WARNING: This page is about a different car, the 2003 Saab 9-3. However, it is still accessible from the selected car via links, so may be relevant.
The control module that is connected to the bus expects data on the C-cable under certain conditions. If the control module does not receive any data it generates a DTC. The fault may not necessary lie in the lead, it may be that the sending module is not energized or grounded.