figure out what the exact problem is that is throwing the Code 21. You do it or he does it. If it's the PTU, and if it's the original PTU, and if Nissan is still warranting it, then it'll be cheap or nothing to fix. If the mechanic figures out it's something else, you'll probably have to at least pay a diagnostic fee. The PTU is originally located in a terribly hot location which plays a part in it's demise. There is a PTU re-location kit available fairly cheap. Routing it is kind of a pain, and if you're good at metal-working or creative you may be able to come up with something yourself. Internet search: "300zx" ptu re-location kit