CEL is on.  New motor been in about 2000 miles.  Code: Bank2 Cat
I have a 3rd cat delete and getting slightly over 15 mpg mostly Hwy.
Good power, by all measures the car is 'running fine or better than fine'.
Asked a Ford guy (from a dealership) and he told me that the 3 cat delete should not be causing mileage issues, even though it will throw CEL; and I should check the fuel trim.
I thought the fuel trim was a function of the computer... How does one check the fuel trim??
Anything else I should look at?

Thanks.

Going to work now, wont be able to answer questions until about 2AM or so (unless I get a minute or so to use the phone)...

I have one of these.  

Autel MS509

It will read short and long term fuel trims in real time.

It's a great scanner for the price.

Fuel trim is calculated using the O2 sensors.

If you get an OBD scan tool, you should be able to read the fuel trim PID, and see if you are running very rich.

You may have an unrelated issue, such as dirty / damaged air sensor.