By coincidence the CEL came on in 2008 Suzuki here.  Code P0420, catalyst system efficiency.  This could be pre-cat sensor, or post cat 02 sensor, or the catalytic converter.

I reset it and it stayed off for a month or so, but it popped on again yesterday.

The scanner I have does see real time data from the sensors and can produce charts.  If either of the sensors had a faulty element, that should have generated a separate code. So at this stage, no need to get my hands dirty.
I reset the check engine light and got the car up to normal temp.  The pre-cat sensor seems to be operating to spec, nice near sine wave pulses within 150-850 mV range.  The post cat sensor signal is fluctuating all over the place and I'm pretty sure it should be reasonably steady approx 600mV.  That's generic - not Suzuki specific.  If the waveform was closely following the pre-cat signal, then I think that's a catalytic converter fault (ie - it's not working). And it shouldn't be a catalytic converter failure as it's a NZ new vehicle with only moderate km. unless some idiot ran avgas in it or whatever.  So I *think* it's indeed a faulty post cat O2 sensor, and I *guess* that the fault code is generated when the fluctuations happen to randomly line up with the waveform from the pre-cat sensor - for long enough so the ecu decides the cat isn't working.  Possibly why you can drive around for weeks or months and the check engine light doesn't come on - except occasionally and randomly.  Anyway I also *think* that the post cat sensor doesn't actually do anything at all engine management wise - it's only there to monitor whether the catalytic converter is working for emissions control monitoring reasons.  If the pre-cat sensor was dodgy, then that could be more of a problem (running too rich or lean).

So after doing that, I'm really none the wiser, and google seems to be telling me that this is a reasonably common issue, and that owners report not great luck or confidence with franchise workshops diagnosing the issue, but end up doing the old parts swappathon at customers expense (great expense if they decide the cat needs replacing).  A new OEM post-cat O2 sensor is $190.

So, my solution for now is to do nothing at all, it's running fine whether or not the check engine light is on - which only takes about two minutes to reset/clear.  I'm not paying $190 for a replacement sensor without being 100% sure, but if it keeps triggering the code, I might try bypassing the post cat signal and feeding the ECU the 0.6v signal I think it expects, then see what happens.