Was thread on here last year about a similar ish issue.
I had the 310; was working it hard in the lakes and cooling fan kicks in - immediately I get the high emissions light and the warning ‘reduced engine power in ■■ hours’ - Scania changed the EGR sensor cleared the fault but ■■ hours later I got the red warning light ‘engine power 50%’.
Basically changing the EGR sensor and clearing the fault didn’t fix it, merely just masked it.
What actually happened was (concluded after many hours in the garage) the engine cooling fan motor spiked the ECU when it came on. Fan was faulty, replced. ECU had to be remapped or whatever it is they do. Since that was done all was good.
However, could just be a wire chafing somewhere…