Heater not working. 2.0 Diesel Business 2016

Hi all. Heater stopped working on my 2016 Yeti 2.0l diesel (Business?). Fan OK. Water temperature gauge shows normal 90 deg. I asked garage to do diagnostic to see whether one of the sensors might be out. Came back with:

Engine management
P20EE00 SCR NOx Catalyst efficiency below threshold. Bank 1.

Air Con
B10AF04 Air quality sensor defective (doesn't clear)

From what I have read the air quality sensor senses bad air outside the car, and switches the air recirculation to interior.
The NOx error is a bit of a worry, but as yet has not resulted in any EML on the dash.

So none of these errors points to a problem with the heating. 

It has the dual climate control (climatronic?). I tried the flap calibration process. No difference.
I read a thread about a sensor in the drivers air ducting. Wouldn't this have appeared on the diagnostic?

Is there anything I can try to pin point the problem?

The coolant level was a bit low. Still above minimum on the expansion tank. I added a bit. No difference. Could it be an air lock in the heater matrix?

I thnk (I could be wrong) when it was working before and I want to demist the windscreen, it would only run the fan at slow speed, even though it was set to high speed, until the car warmed a bit. Now it runs at full speed. Another sign of a dodgey sensor?