Maybe the cat has actually lost it's effectiveness. Aftermarket ones are nowhere near as good, or as long lasting as the factory ones. It's a gradual process.
On the factory exhaust, you can't tell if the front or rear cat is knackered because the rear lambda sits downstream of both of them, so visual inspection (or laser thermometer) is the only way to tell. But as your BCS cat is the only cat in the system, and your map is standard, it's the most likely answer I'm afraid. Occam's Razor and all that
I have an APR downpipe. Expensive, and the sales pitch about their amazing "GESI" cat is laughable. In reality their cat is utterly useless. My car only just scraped through the MOT with 0.1 emissions. It should be 0.01, or zero. That's how it is with some aftermarket cats.
