W205 2014 engine management light

[Andyp1854]

Hi
New to MB hoping someone can help
2014 w205 c220 d
Driving home Friday engine management light came on plugged in a cheap reader and it came up with code P13DF. What I can figure out is low Adblue but no warning light on dash, I put about 17 ltr in, cleared light but comes back on ? Gave it a 30 mile hard run and left it running for a hour
Restarted and light gone of went back out 1 hour later back on
Can anybody shed any light or answers please

 

[ioweddie]

nox sensor

 

[Andyp1854]

Did have a problem with a box sensor last year when I bought it but garage supposed to have changed it
Looks like they just knocked the light of

 

[Andyp1854]

Adblue warning just come up on dash
Engine shutdowns in .... miles

 

[SMS]

There are 2 nox sensors and have been modified several times but there are a number of faults that will trigger that warning.