13' 35d DeNOx low efficiency
 

My 13' 35d has been throwing the following codes, I've cleared them and they continue to return after 100 or so miles.

DeNOx system efficiency efficiency at low - 004D18
DeNOx system efficiency: efficiency is too low in the range 1 - 0042FC

Does anyone have suggestions on where to start? I'm new to diesels and DEF.

The NOx catalyzer is in the mid pipe. There is a sensor before and after. I would suspect an issue with the post-catalyst sensor before I suspected an issue with the catalyzer itself. Assuming there are no codes for the urea injection.

No codes for urea, anyway to confirm the sensor is bad? I recently had a NOx sensor replaced under a recall, I'll have to find my paperwork to see which one it was.

INPA or another diagnostic tool with live data should be able to sample the sensors so you can see if it's just flatlined or gone out of range.

Continuing where we left off, I went to the dealer and they flushed and refilled the DEF & confirmed spray pattern was good. Today were getting an incorrect DEF fluid, 200 miles until no start. This is the first time I have gotten this error and it seems to be a result of the flush\refill, do I dare go back to the dealer? Anything I can do to confirm besides checking that they filled the fluid?

That error could easily be caused by the same sensors. I would insist that they replace the pre and post cat sensors if you have a warranty.

I'll have to confirm the warranty status tonight.

Check out post #7 here:

Incorrect Exhaut Fluid Message! - Bimmerfest - BMW Forums

Possibly is the catalyst. I guess they have been known to go bad. But I believe the sensors are much cheaper.

That would be a heart breaker, any chance the SCR cat is covered under warranty?