Mercedes B180: engine warning light is on, probably dodgy cable?

I have a Mercedes B180 Diesel, from 2013/2014. The little ‘engine warning light’ came on, I took it to a garage, where they said it was a faulty oil pressure sensor, so they changed that. The light remained on, so now they say it is likely to be the cable; but when I asked them to change it, they weren’t too keen – apparently the exhaust dust filter (forgot the proper name for it) needs regenerating, and they believe that this may be the reason this light comes on.

Does this sound likely? And why hesitate with changing the cable? Unless it is one of these things where all cables in the car are integrated in such a way that you have to change all of them, and it gets massivly expensive.

Finally, I’d like to find out for myself about this cable; is there anywhere I can findout, like a database, or does somebody here happen to know what it costs or the partnumber perhaps?

Sorry for rambling, I’m just a little fed up with this. Modern cars are too complicated – and unapproachable, not least because it seems impossible to get any real documentation.