Engine management light, temperature up and down

[Jimmy2007]

Hi everyone,
The engine management light came on and I noticed that when I go on motorway the temperature drops.
If I go on manual and leave the 4th gear on, the temperature drops to about 75 degrees, but goes back up if i go on 6th gear but not all the way to 90.
If i'm in traffic it stays on 90.
I noticed for a couple of months that it took quite a long time for the car to warm up, about 5 miles of urban roads.
I'm thinking thermostat or temperature sensor, but this is purely based on my experience with my VW passat.
Probably totally unrelated, but last friday I collected the car from Audi that fixed a dent in the rear bumper and changed a parking sensor following a small rear end in traffic.
What do you think?
It's an Audi A6 C6 4F 3.0L TDI quattro without dpf.
Thanks

 

[Jimmy2007]

Update.
After a 20 mile stretch on the motorway the light has disappeared.
Any ideas?

 

[abmat]

Without a scan I’d be clutching at straws

 

[Jimmy2007]

Without a scan I’d be clutching at straws

Yes, I know.
Is it possible that there's an emission sensor that hot dirty and after a blast on motorways at high revs it self cleared?
Seen this on YouTube for a Q5, don't know if it applies to my car.

 

[Garrett 69]

Isn't there a sensor just after the dpf? Possibly as you say, it was just sooted up.

 

[Jimmy2007]

Isn't there a sensor just after the dpf? Possibly as you say, it was just sooted up.

My car hasn't got a dpf, but reading online I think it's got an emission sensor.
Can anyone confirm?
In the meantime I also read that when running in manual mode on high revs (4th gear at 70mph) the coolant is pumped around to avoid overheating and the temperature drops.
Then when in 6th gear, it goes back to the usual 90 degrees and stays there.
Possible?