I think you have answered your own question.
You need some diagnosis.
Either take it to a local indie garage, or main dealer if they are not going to charge a lot for diagnosis. Or get yourself a cheap F2 diagnostic tool, I got the Autel...
my emmisions are lower than a hybrid, i have a TD5 with 130K on it and an Alive stage 2 tune, DeCat/DPF thingy removed stainless exhaust, and no EGR, how is that possible?
Regular use of Millers Ecomax and a good blat down the motor...
I THINK I FIXED IT! (Long post)
I took a look today before it started throwing it down and found a couple of things;
Firstly I took the distributor cap off to inspect for water ingress from greenlaning and found OIL inside the distributor cap...