Other option is a code reader /resetter that will reset the codes on the few occasions they appear ... around £80 for a decent one .. cheapo crap off ebay less than £20.
I reset mine twice a year it seems ... and it takes less than 2 minutes (including stopping, finding reader, doing the job, putting reader back, driving off..)
Oh, so the sensor thing is just a bug? It's not actually faulty? Fair enough then.
Its sort of a fault.
Modern cars (ODBII compliant cars) use a second sensor to measure cat efficiency.
On an Omega, these 2nd sensors (V6 has 2, one per cylinder bank) are after the "starting cat", but in front of the main cat, so in effect is measuring the efficiency of the starting cat, not the main cat. On the 3.2, these starting cats do degrade, thus flagging the cat efficiency codes (P0420 and P0430).
Solution, move these 2 sensors behind the main cat (involves cutting in a hole, and welding a boss in, to screw the sensor into)