Can my day get worse? Engine Management Light on!

Hi all, its that annoying NOOB again, with another query. Hope someone can advise or has suffered similar, thanks in advance.

We took the new (Secondhand 1.2 K12) Micra out for a spin today after purchase on Thursday. About 20 minutes in, the EML came on as we went around a roundabout to join the dual carriageway. No noticeable power loss, noises, smoke, bangs, knocks, etc. The car carried on driving fine, another 7 miles to our destination. We got there and I stopped the engine, restarted and the light came on again. We did our stuff and headed home, again, no obvious degradation of power or any other symptom.

We reached home and luckily I had an OBDII ELM327 reader in my drawer. I connected it up and ran the program on my phone, fault code listed was PO420 (Catalyst Efficiency Below Threshold (Bank 1)

Doing some online checking, it gave me various options: Faulty Cat/Faulty O2 sensor pre or post cat/Air leak/Exhaust Blow, etc

I thought I could hear air hissing/blowing from somewhere when it first set the EML off but had a look under the bonnet and in engine compartment whilst wifey gave it a rev, but didn't hear it again.

Took a snapshot waveform of volts/time on Bank 1/Bank 2 O2 sensor (attached) 3 separate snapshots.

I gather the front sensor should pulse about 5 times in ten seconds up to 0.9 volts, the rear sensor should give more of a smooth graph plot, with less oscillation.

On checking both graphs over the samples, they appear to be ok, but there are sudden dead spots and breaks in the track.

Would any of you have any experience of this or opinions on the info supplied?

The guy whom I bought the car from has given me a month warranty, so I will take it back to him on Monday, but I would like to have a better idea of whats likely to be wrong in case they just reset the EML and say "Nothing Wrong"??

Many many thanks in advance for your efforts.

Ian (Cyclerider)


Update: Trip to Garage today, they wired her up via the diagnostics port and used their "very expensive" gizmo, to ascertain a "Fault with 02 sensor on Bank 1" So they have reset my fault codes and ordered me one for fitting later. Hopefully a cure.