I know that a frequency valve is used on volvos at least early models with CIS injection system. It is a pulse width modulated valve that is used to control the air / fuel mixture and is driven by the Lambda computer. MB uses a device called EHA (electro hydraulic actuator) to do the same thing. I question if your mechanic really knows what is wrong or maybe he is trying to baffle you with bull----. You didnt say what the problem was or what type of car it is.
You tell if it is bad by checking to see if the on-off ratio is working correctly and is in closed loop mode. If it is regulating correctly everything is OK otherwise it might be defective but other things in the system can cause it run open loop.
I got too many cars!! Insurance eats me alive. Dave
78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k