There is a vacuum line that can come off that is difficult to see. I have 96 850 and a 98 S70 and this same line has come off on both cars resulting in the check engine coming on. This vacuum line is located on the far left side of the intake manifold at the #1 cylinder. The vacuum line goes to a servo hidden underneath the intake manifold. You have to squeeze your fingers between the thermostat housing and the intake manifold to put the line back on. You may need to get a pair of needle nose pliers to do the job. The other end of this line connects at the flame trap at the entrance of the throttle body.
Go to an Auto Zone and ask them to check your OBD II codes. If you get a code that says "right bank lean" or something like that, the tube I am describing is more than likely the cause.
Good Luck!
GP
93 940 175K
96 850 110K (My favorite!!)
98 S70 85K
74 914 Porsche