It depends on how it is monitored there is the possiblity for the engine to run away with itself. In this case the Turbo started sucking oil from the sump and the engine was unning on oil, this is what goosed the turbo as it needs that oil to lubricate.

This will not result in a fault code but a good mechanic would be able to spot this by monitoring the engine read out. The exhaust gas temps can reach up to 800 deg C during a regen which is how it burns off the soot in the particulate filter.

This is not my issue with the garage however it is that they incorrectly diagnosed the reason for excessive white smoke after the regen when they road tested the car, and there was virtually no oil left in the sump, when they returned the car to me after the work.

When I give a garage a car with oil in it and it comes back with no oil something is wrong, and if a mechanic sees excessive engine smoke i would assume that the 1st thing he checks is the oil (its the first thing I checked after driving the car 2 miles home). The effect of this could have been a serious incident on the road if the engine had seized up, as their customers we put an incredible amount of trust in garages and it the work is not up to standard and they fail to spot a potentially disasterious fault in a car before returning it then they should be held to account.

It's a tough one this is. If your oil was low, does that mean it was low when it went in? I can't see why the garage would have drained your oil and in my opinion, it's your responsibility to check your oil every so often, especially on a diesel, seeing as they usually munch a bit. If I took my car into a garage for some work, I wouldn't expect them to top my oil up for me if it was low.