Ssangyong Tivoli engine management light and slight loss of power

I've just bought a 2016 1.6 diesel (had it literally 3 days)the engine management light has come on every day with a feeling of touching the brake slightly while driving and slight loss of power and the engine suddenly sounds like a clapped out taxi,it's been back to the dealer almost daily and each time he puts it on diagnostics and the first time it didn't bring any fault up but the 2nd time he didn't actually say if it had or what was wrong with it he just said it should be sorted but the next day it's back on.i love the car but I'm losing confidence in it.does anyone know what it could be?it's getting tedious returning it daily. Thanks