1 Series engine warning light

[gfs]

I have a 08 1 series 118d m sport. Last week had the engine warning light pop up. No loss of power car felt fine. Took it to BMW who replaced engine coolant pipe although not entirely sure if its exactly called that as I'm pretty useless with cars. anyway 100 miles later the same engine warning light comes on. It's done 50000 miles any help would be appreciated