I understand that the 'root of the problem' is in fact that the systems were designed and built to operate with chemicals which are no longer available. Somebody has already given the explanation that the current products work less efficiently than the original, hence the problems. Of course, maintenance also plays a big part, and we know that at least one TOC manages to get the air con to work rather well.
I must say I think the insistence by some that the 158 is a badly built train is clearly cobblers. They are 20-odd years old and still going very strong, and are of such a quality that SWT at least manages to present them just as well as any new rolling stock, and in fact happily exchanged nearly brand new Turbostars for more of them. The fact that other operators just run them into the ground is hardly the fault of the train itself, and applies just the same to anything. Where does this idea come from that somehow, despite having been endlessly churning out fleets of perfectly decent new trains for decades, BR got to the end of the 80's and suddenly built a couple of hundred crap DMUs? I think not, they're perfectly solid trains. Is the new stuff any better? A Turbostar, or an Adelante, or a 458? Anyway, rant over!
As for the 166, it's hardly fair to suggest that they were 'cobbled together by reworking the 165 design'. They are two of four different classes of the Networker common design family, along with the 365 and 465. A further, albeit very similar, development of the original 165 build perhaps, but hardly a bodge job!