Have a 2007 s40 t5 and at 4000 rpm the power falls off. Not sure if it is a fuel problem or turbo. Last week this happened and it went into reduced engine performance. Turned it off and on again and it has not done again. ?????

 

Any trouble codes?

 

check engine does not come on at all. should i still scan for codes?

 

No check engine light then no codes IIRC. How about revving by hand? The newer S40's have drive by wire throttle control so I'm thinking there may be a dead spot in the throttle control? Something similar to a dead spot in the throttle position sensor causing problems in cars with a throttle cable.

Also, have you looked to see if all the underhood piping is intact? A leak after the Mass Air Flow sensor could cause problems.