Hey i have Volvo 2013 D13 and have SPN 3216 FMI 12 code

So from google search i have learned that it's nox sensor and was wondering if anyone can tell me the function of nox sensor because I have been driving with check engine light on and everything seems to be working normally with DPF system actually the light came on and the code spn 3216 fmi 12 and when off few days later and now it's back on again same code same everything but everything it seems to be working normally with DPF. The soot level is really low and it uses def normally like it has always been using. So what seems to be a problem here anyone else had a similar problem?

 

That is definitely what a computer telling me but everything seems to be running just fine no high soot level. Been running for a week now with check engine light on with a soot level only at the quarter and at times even less then a quarter

 

Filter was cleaned at 334000 miles so it's been only about 70,000+ miles since last time cleaned. The first time check engine light came on I played with harness little bit and reset the check engine light by turning key on and off 5 times and on a sixth turn I started the engine and check engine light and code was gone next day. About week later I check my soot level and it shows hardly any soot and that same afternoon check engine light is back on and soot level is at quarter and it kind of stays that way it goes down a little bit come back up but don't go pass quarter so I don't know if there is a wire loose somewhere or I really need to replace nox sensor ?

 

That's what I was thinking thanks for help.

 

Never been replaced