What is the F hose?

 

The rubber hose that both Diverter Valves plugs into (kinda looks like an F).

 

What about the one below the DV's, that is Y shape, has anyone found that one screwed up other than me?

 

I am having same symptoms and fault codes on my new-to-me 2001 996TT as OP cited in post #1 some 12 years ago:

Though in addition I am experiencing weak boost (0. 2 or 0.3 max bars). I have not looked at the collapsed hose that OP experienced, and have not yet performed boost leak test. But since symptoms and codes so similar to mine, I was curious what OP's resolution was?

First post here on 6speed. 20yr 996 NA owner, new to 996TT last month.

 

testing will reveal the leaks you probably have. a collapsed hose is massive and would result in the issues and absolute loss of boost as you describe and outlined in your other thread - while a minor leak ( like say a faulty #16 check valve ) would also cause issues of a lesser severity. but leaks are leaks - and you've probably got some.