10 days ago the boost on my 996 GT2 dropped to 0.5 bar,i took it to the dealer,from where i bought it 7 months ago.they gave me 1 year warranty.they did the diagnostic for $35 and told me that the Y piece is dammaged and they have to purchase it from Germany because they dont have it in stock ,and i have to pay the part and the shippment because it is not covered by the warranty.After 4 days they called me and did say that thy receive a wrong part ,so we have to wait 10 days .Is it true that not all parts are guarantied??
Excuse my english and thanks

You need to read the terms of the warranty you bought to see what it covers. They're all different.

For reference, the warranty I have (Fidelity Platinum) covers everything bumper to bumper with a $100 deductible. I had a leaking oil pump seal and a bad head unit that were both replaced ($5500 for both) for only $200 cost to me. But this is one of the best warranties out there.

Could you give me more info on the warranty you purchased? Price and how long it last? Also, if you purchase the aftermarket warranty and then mod the car any after getting it, will that void the warranty? Reason I ask about the mods is because my buddy has a modded E55 and its still covered by the aftermarket warranty pretty much because they didnt know it had been modded.