Polo Cat emissions.

I can't recall ever thinking this one through, and I've certainly never been afforded the opportunity to diagnose this setup.

A 2004 Polo with TWO CATS fitted and two lambda sensors. Bank 1 sensor 1 and Bank 2 sensor 2 (in series)

The boss failed it on emissions a few days ago. After he had read live data using his Bosch scanner and watching the lambda gas reading going up and down on the gas anaylser he decided that the lambda sensor Bank 1 sensor 1 was faulty. I knew nothing at this time except I was asked to change the sensor.

Having removed the bank 1 sensor 1 from the front cat my visual inspection told me that nothing was wrong with the sensor. I was told to replace it.

Then another exhaust emissions test was carried out by the boss and he said the lambda gas reading is up and down and this is due to a faulty lambda!

I was again asked to look at it and check for engine air leaks. Having found nothing I decided to do an emissions test myself. I decided from the gas readings that the cat was inefficient, but which one?

The boss didn't agree but today got the other guy to fit a rear cat. Another emissions test and it passed, but is the front cat efficient, or is the second cat now working too hard to maintain standards?

If a scope was used on this setup, which cat would be identifed?

I'm sure a lamda and section of exhaust would need to be installed between the cats, unless somebody knows another test method!