Focus MK4 2019 1.5 Tdci( no adblue tank) Engine ZTDA.....72k .' NOx trap failed. Efficiency below threshold error' and eml comes up after various numbers of cold starts ( 5 to 38)
Attempted to read NOx sensor reading(s)( physically found 1 sensor at top of filter...maybe a 2nd after cat?) but these do not appear as available to read on this vehicle!. They appear in the 'all sensors' list but not on 'current vehicle ' Tried using ' Car scanner' and ' obd fusion' android apps and 2 obd adapters( Bluetooth and WiFi). Most Other readings seem available and ok ( although a few are blank eg miles since last dpf regen). Have I missed the obvious ( quite likely).I have read there is a ECU update for early engines relating to NOx. No idea if this has been done. Can version number be read?. Is this a main stealer only update?

Bit of update 

Discovered the error is showing as a fail under 'non continuous ' monitors  most of the time and when it passes it is in upper region. I just need to actually locate the sensor next!. Any idea where noX sensor is or how many there are?

The NOx sensor looks very similar to a lambda sensor but connected to a black box with a short loom.  There are likely to be two.  One before the NOx trap to measure engine NOx.  Then another after it to check the NOx levels are being adequately reduced before reaching the tailpipe.

The NOx trap is an extra box in the system that looks like a small cat.  Sadly we don't have access to parts catalogues for the Mk4 so you will just have to look at yours unless anyone can offer better advice.  

 

I've pulled this picture from an online ad, showing the NOx trap plus one NOx sensor from a Mk4.  Though I don't know if this is the exact setup for your car.

Update 2.

Found a NoX sensor exactly as Toms pic....what I thought was the 1st sensor is actually a O2 sensor so need to dig deeper to see if I can find a pre absorber Nox sensor. Not sure how much help it will be as being unable to read( or even see ) the sensor on obd makes diagnostics pretty much impossible....wether they are mapped out I am not convinced but certainly not showing as available. Also noted the non continuous monitor that shows the fail reads one of 2 readings....very precise.....always identical.....one a pass(86.621864%)... Just in... or a fail ( 94.999604%).... That's weird....surely they should be more random?