You can do a lot of checks yourself.
* diverted valve:
Its a small black valve that located near the top right hand corner of you engine bay just next to your air filter box. It kind of looks like a small round tube with 3 tubes leading off it. One on the the top (very small) one at the bottom and one on the side (both of these are the same size) it will take you about 3-6 mins to remove it and all you need is a driver to undo the screws on the clips holding it in place.
Once you have removed it you will notice that if you put your finger in the bottom you can push up the piston (it should be hard) now what you want to do is push it up then before your remove your finger cover the nipple tube on the top then remove your finger from the bottom. If done correctly you will notice that the piston you pushed up will stay up (note that its normal if it moves a tiny bit). If however after creating a vacuume the piston just drops straight back down then yes your diverter valve is broken. But if it stays up its fine. Also worth noticing if you spot small amount of oil in and around the valve, don't panic, that's normal.
However when remapping a golf you should never really use a stock DV you should always upgrade to a forge 007p or forge 008 and always ensure you use a recruitalting DV and NEVER a "dump valve"
Hope this helps
