Re: P0430 Fault
passing an emissions test doesnt mean the cat is working ok (i've passed without a cat fitted before).
the lambda sensors compare the exhaust gas before and after the cat, if the change is not large enough the cat is deemed ineffective and the light is triggered along with the p0430 code.
if the lambda after the cat was faulty then the code you get would not be p0430. obd codes are not "general"; there are loads of them and p0430 means only 1 possible thing, the sensor after the cat is detecting very similar results to the one before the cat, so the cat isnt doing very much.
the only way you can get this code without haivng any fault is when your emissions are so good that the cat cant improve them (not as unrealistic as you'd think)
since it is still at the intermittent stage, you can usually clean your cat to get a couple of years out of it. best way to do that is add 2 full bottles of redex (or a pint of kerosene) to a quarter tank of super unleaded, and then give it the drive of its life for at least an hour, keeping the rpm high. if you detect an eggy smell from the exhaust afterwards you've probably done the trick and the light wont come back on.