You don't state ignition timing or engine coolant temperature. As you said, high engine combustion chamber temperatures are the cause of high NO readings and both of these can raise it enough to fail. I know they are watchfull where you live of tricks that some people have used to pass so I'll skip any of them, like rigging the radiator fan to stay running or the heater to be stuck on high. So make sure the timing is set to the low side (not advanced more than the book and maybe a tad lower) and make sure you not running too hot. You also have to come to terms with the possible fact that a CAT ages and could just need replacing.