The codes for sensor malfunction are usually caused by unmetered air getting into the intake manifold. This causes a lean condition at the front O2 sensor and the ECU compensates by increasing the fuel injector opening. Your MPG then goes downhill, the O2 sensor gets covered in soot and then fails again.

Soooo, you probably have a vacuum line leak, a leaky vacuum tree, air intake before the throttle body leak, and/or a leaky intake manifold gasket. You could also have a broken diaphram in the fuel pressure regulator, allowing fuel to be flowing down the vacuum line to the tree and into the manifold.

Take your pick from the above, and don't forget to clean up the PCV.

Klaus

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Proud owner of a 1967 220S. If I had more room, I would have more Volvos.