P0420 code - O2 sensor or cat replacement?

[slipshot]

Peeps

Has anyone else experienced the P0420 error code and how did you fix it? My car NB8B (2003) is showing one and I'm trying to determine next steps.

It first showed it about 4 months ago so I purchased a simple extension piece and used this to take the current rear O2 sensor out of the air flow. The light went off when I reset the DTC's and it was fine. It ran like this for about 1500km and suddenly the other day popped up again. Same code...

I know the car has done 153,000km and the cat will not be great, but I'm also conscious it could well be the O2 sensor. Basically, as a replacement cat is cheaper than an O2 sensor, I;m trying to determine which I should replace first. Or if there is a simple way I can check if the O2 sensor is performing correctly?

Any advice would be great? Thanks

Andy

[93_Clubman]

Basically, as a replacement cat is cheaper than an O2 sensor, I;m trying to determine which I should replace first. Or if there is a simple way I can check if the O2 sensor is performing correctly?

Have only checked MX5Parts in UK, but suspected O2 sensor would be about half the price of a cat: O2 sensor: GBP59 v cat: GBP100.
Perhaps not a simple test depending on the equipment you have:
http://www.miataturbo.net/ecus-tuning-5 ... est-48158/
http://www.miata.net/garage/KnowYourCar/S3_O2.html