Citroen GP and P20EE DTC (NOx)

[perfidus]

It's an old post but I think it's ok to be revived.
I have the same P20EE 92 and P2BAD 92 NOx errors - I've got them soon after a DPF cleaning and NOx sensor replacement, but I just get rid of them with the NOx sensor replacement/learning procedure in Diagbox.
First attempt to run the procedure failed because there was an adblue issue, and I left it unlearned.
Although only changing the NOx sensor without learning procedure might work for a while, after driving around 150Km the errors appeared.
In the end I resolved the adblue problem and run NOx sensor learning again. It takes almost 30 minutes to complete the procedure with engine on and revs about 2200 rpm.
After that I drove about 2000Km and no error returned.