Boost Pressure Control (BPC) Valve
WARNING: This page is about a different car, the 2002 Volvo C70, 2001 Volvo C70, 2000 Volvo C70, 1999 Volvo C70, and 1998 Volvo C70. However, it is still accessible from the selected car via links, so may be relevant.
The boost pressure may become excessive if turbine speed is too high. This is prevented by the boost pressure control (BPC) valve (5), which bypasses a proportion of the exhaust gases flowing to the turbine.