Volvo: 99 S70 GLT Check Engine Light, volvo dealer, new spark

Question
My local mechanic cannot seem to fix the problem w/ my 99 S70 check engine light.  The codes always seem to indicate a bad O2 sensor.  These have been replaced several times, as well as  the installation of a new air flow modulator, new spark plugs, and troubleshooting the electricals. The light comes back on after about 100 miles after a new O2 sensor is installed.  Any ideas?

Answer
Derek,
If the new H02S were not from the Volvo dealer, they might not be the latest updated style. Some also come with a short wire harness. If the H02S are correct then something in the fault tracing was missed. You might go to a shop that works on Volvo and see what their solution is.