Check all of the Vacuum Elbows. The primary suspect (if it's one of these) is found under the t-stat housing coming off the intake manifold and connecting to a hose that runs down under the engine. It can be hard to see (use a flashilght) and even harder to get to. I just replaced mine. Often, the heat and grime will cause the rubber to go soft and either collapse or open a hole.

As was mentioned, be sure to get the codes read. Go to Autozone, they'll do it for free. It will be the generic code, not the "Volvo code," but it's a good place to start.

Sam