On the Ciera you can improve access to the rear cylinder bank by removing the plastic cover over the ventilation fan. This is held in place by just a few screws on top. It may be different for a Silhouette but it may be worth a try.

What model does it concern? I know that the XF 2.8D models get this message when a (completely unrelated) valve in the diesel particle filter fails, but this may be an entirely different issue.