P0420 and P0430 on X5

[spandit]

Hello BMW owners

I have 2002 X5 3.0 . Sometime back SES light turned on and when I checked the code it was P0420 and P0430 .

My mechanic suggested replacing O2 sensors as it was already 88000 miles. After replacing O2 sensors the car went fine about 1000 miles and I got the SES lights turned on again . Same codes again .

The car runs absolutely fine with the exception of poor gas mileage . Around 16.2 City .

So My question is what tests can I do to verify if the cats are working fine ?

What could be other cause of these codes and how to rectify ?

Thanks
Sunil