Since my purchase of my 2012 Passat TDI, used at 22,293 miles in May 2015, I too have had issues with the NOx sensor. Below is my history of flags I cleared vs time and date:
Date Milage Code No.
5/23/2015 22660 15733 - SCR NOx Catalyst Bank 1 - P20EE 00 [236] - Efficiency too Low
8/26/2015 25483 18627 - NOx Sensor 1 Bank 1 - P2200 00 [096] - Electrical Malfunction18627 **&** NOx Sensor 1 Bank 1 - P2200 00 [096] P20EE 00 [172] - Efficiency too Low
9/5/2015 25920 15733 - SCR NOx Catalyst Bank 1 - P20EE 00 [236] - Efficiency too Low
9/9/2015 26222 18627 - NOx Sensor 1 Bank 1 - P2200 00 [096] - Electrical Malfunction18627 **&** NOx Sensor 1 Bank 1 - P2200 00 [096] P20EE 00 [172] - Efficiency too Low
9/21/2015 26687 18627 - NOx Sensor 1 Bank 1 - P2200 00 [096] - Electrical Malfunction18627 **&** NOx Sensor 1 Bank 1 - P2200 00 [096] P20EE 00 [172] - Efficiency too Low
1/2/2016 29653 18627 - NOx Sensor 1 Bank 1 - P2200 00 [096] - Electrical Malfunction18627 **&** NOx Sensor 1 Bank 1 - P2200 00 [096] P20EE 00 [172] - Efficiency too Low
The September readouts that popped up frequently was during my attempts to clean the sensor. As you can see I had a good 3+ month run for 3k miles before it popped up again on a recent 500 mile weekend trip.
Yesterday I performed my 30k oil change and reset the code. I'm hoping the fresh oil will calm this down. The care was service by a VW dealer before my purchase, but I can't vouch for the oil used then. I opted to use the Quartz INEO Long Life ACEA/API 5W-30.
My initial response was to buy the expensive sensor, but as I've read that cleaning may solve the issue, and oil could be an issue. The car consumed 2.25 gallons of Urea in around 7500 miles.
I will post if and when these errors pop up again.