MAF or cat?

The P0138 code is not the MAF sensor. It indicates that the Heated Oxygen Sensor #2 (bank 1) is not responding properly. My SEL came on with a P0140, a different sensor and my mileage dropped about 3 miles per gallon on the average. Before I could get it to the shop, it went out. It runs fine and the mileage varies.

I'll keep watching to see more answers.

You need to get the code read again. It might not be the same problem as before. Many parts stores (e.g. AutoZone) will read it for free and tell you the number. Somebody here should be able to tell you what to check once you get a current number.

any readings on the intake air temp, MAP sensor?

It could be the EGR valve.

PickleJuice wrote:

I found that if the assembly that holds the MAF becomes loose from the throttle body, the car will run terribly or not at all. A quick shake will tell if it's tight as it should be.

It's a cylindrical device inline with the air feed to the throttlebody. It is connected electrically. Here is a picture of it. If you need to replace it, this is where to buy it for the 2000 Max.