UPDATE: 11/22/2015

3 weeks after I went through car wash forgetting to close my diesel fuel cap (fuel tank was full at the time). I filled up my tank to full after it dropped to 1/2, no problem at all. Now it dropped to 1/2 again after total of 400+miles of driving since the car wash.

Engine light came on today. I scanned it with OBD and gave me following codes:

P2201 NOx Sensor Circuit Range/Performance Bank 1 Sensor 1
P124D <--- scanner does not show what this code is
P20EE SCR NOx Catalyst Efficiency Blow Threshold Bank 1

So I took the Nox Sensor out of the SCR under the car, and there were some water at the tip of the sensor, I guess it is not normal. I cleaned and put everything back. Reset the codes (engine light gone).

After driving it to reset ecu and driving cycle, P2201 and P124D shows up as pending and P20EE as permanent. No engine light yet (I think it will come on soon).

I have searched online and many people have replaced NOx sensor ($500) and SCR ($1000+) at dealer.

Do you guys have any idea how I should fix this?