[glemmy]

A while ago I got a warning message in the display cluster, Adblue system defective.
At this time the level in the Adblue tank was low. I went straight to a gas station for a refill. The error message disappeared (but it probably would still have disappeared, had the tank not been refilled).

I went on to drive about 1500 km and then the error popped up again. Then disappeared and another 1500 km it was back again.

The fault codes are:
280F00 NOx sensor after DeNOx-Kat, signal NOx: NOx signal invalid for too long
280D00 NOx sensor in front of DeNOx-Kat, signal NOx: NOx signal invalid for too long
26F500 DeNOx system, efficiency: efficiency too low in range 1


Today I ran a diagnostics test with Ista+ which checks the NOx sensors.
It looks fine, I drove for about 20 minutes. The values vary from a few PPM up to 450+ ppm or so during heavy acceleration, both sensors seem to respond, values keep going up and down.

Any suggestions what I can try next?