I had a fault once with my 02 wrx, can't remember the exact name for it but basically at high revs under acceleration it would sometimes throw a code ( before hitting redline ) something along the lines of crank angle sensor fault ( from memory might not be exactly correct )
A Subaru dealer told me that it was a common fault on my car and the best way the fix it was to get a tune and to have the values changed of the limit of the sensor. So it will still be detecting but only throw a code at a higher out of normal level.
Or the fault light would go away after a few starts if I wasn't too bothered about it. ( 10 from memory )
The Subaru dealership referred me to MMS and to get a tune. As MMS would be able to do more with the
ECU then what they could and had more experience in doing so.
Now I don't know if the where meaning a FULL tune or just a little tweak on the sensor only.
So in my experience I had a dealer tell me to go get a tune.