**UPDATE**

Brought the car in a few weeks ago. The problem was a "DEFECTIVE OVERFLOW CONTROL VALVE". They replaced it and 1000 miles later the error message has not shown again.

BTW, just did a round-trip LA-San Francisco and got 24 MPG door-to-door with an average speed of 76 MPH. Diesel #1 going up, Diesel #2 going down, with no difference in MPG. Given that I did some passing of cars and was up to 95 MPH at times, I'm happy with the observed fuel economy.