I've had exactly the same DTC on my '94 850. I've put new throttle position sensors (TPSs) on twice: the code returns after a couple of weeks each time. If replacing the TPS works for you please tell me what you did, since the Volvo dealer hasn't diagnosed the problem either.

Locating the TPS is pretty easy. Follow the 3" diameter corrigated black plastic hose from the air cleaner to its end at the intake manifold. The hose terminates at the throttle body. The TPS is mounted on the the front of the throttle body -- the side nearest the radiator. The TPS is a black plastic rectangular affair that has an electrical connector on the side nearest the air cleaner. Two allen cap screws hold the TPS to the throttle body; it is actuated by a keyed shaft that only fits the TPS in one way.

Replacement is simple and safe. Don't overtorque the allen cap screws. Put dielectric grease on the connector. And clean all your engine grounds.

That said, the A6:411 DTC implies that the signal from the injection control to the engine control is faulty. According to the wiring diagram, the TPS connects to the injection control unit, which then connects to the engine control unit. It is possible that the problem is not with the TPS but rather with the wiring, the injection control, or the engine control. Good luck: my Volvo dealer wants me to buy replacements for the expensive control units. I'm hoping that is not the problem.
--
-- Bruce / '82 244 (323k miles!), '86 745, '87 760Ti, '94 854