2008 toyota yaris 1.4 d4d engine

[Coppard 90]

Wonder if you can help my mums having issue with her car its had lose of power for little while but the other day the engine management light come up it come up with supercharger overboost control po234 fault code  had it in the garage the mechanic has checked turbo and wastegate and the vacum solenoid and cant find the problem he seems to think might be wiring problem and has suggested I get a auto spark look at it just wanted to no if anyone has had similar problems or knows what I should do next? Cheers Steve