If the engine experienced the "run-away" scenario, then it was most likely down to the oil level being too high (where was it on the dipstick the last time you checked it?) or a turbo seal failure (as has been said).
Either it was too high to begin with, and the two or more regens diluted the oil with enough fuel to tip it over the edge, or the garage may have been sneaky and reset the oil degradation counter without actually changing the oil, causing it to thin-out too much and cause premature engine/turbo wear before eventually failing. A check of the parameters may help to determine this.
TBH, Fiat's DPF system is very good compared to others, but can be let down by the occasional sensor fault or the lack of info given to owners by the Salesmen (for fear of scaring them away). I've disclosed Technical info in the past to help Forum members and dispel a few myths- one being that it needs to be thrashed on the motorway once a week/month (it'll happily do a regen at idle- as long as you recognise it's doing it and don't switch-off before it's done
), for example.
I had one which kept regenerating on a far-too-regular basis before. A few minutes reading the ECU parameters identified the Exhaust Gas Temperature sensor was reading no higher than 40 degreesC, even during a regen where it should be practically glowing! This meant the ECU believed the filter wasn't getting hot enough to clear, so it repeated the procedure over and over, not recognising the sensor readings being incorrect. By the time it came to us, the damage had been done- it had spun a shell on number 1 con-rod due to the thinned oil. Replacement engine fitted, and a new sensor fitted and working correctly- and all was perfect and we never saw the car again for apart from usual servicing.
What I'm getting at is that you need to find out the cause of the failure before you fit the new engine in case the DPF regen issue was a symptom, not a cause.
