Assuming that the car was for prolonged time in service with an damaged turbo line I think of the following possible scenario:
Normally a diesel is running lean (more oxygen than fuel). Since the split turbo line allowed metered air to escape, the combustion went rich (less oxygen than needed), this way the exhaust gasses where rich on soot. The large amount of soot over prolonged time can cause multiple problems:
- The EGR valve could be clogged. Uncertain: This should definitely throw a CEL with corresponding code.
- In the long term the intake manifold and other engine parts could suffer from soot-buildup.
- The engine oil is contaminated with an larger-than-normal amount of soot.
- The heavy amount of soot in the particle filter provoked often a filter-cleaning cycle (Fuel gets injected on the exhaust stroke, to burn off the soot in the filter. Detriment is the contamination of the engine oil with fuel)
My thoughts about the most likely cause:
I assume that the "service-soon" signal is caused because the fuel proportion in the engine-oil is too high as there where too many cleaning cycles. The engine counts them, as each cleaning cycle dilutes the engine oil. Now the engine oil is diluted up to the point where it needs to be replaced. Fuel diluted oil has worse lubrication quality than non-diluted oil, therefor the ecu activates the limp-mode to (somewhat) limit the damage to the engine
Book a service (oil-change) for the car as soon as possible. Ideally you do this at a place where they are able to do further diagnosis (no quick-lube booth, perhaps the dealership or a reputable garage) as further investigation could unearth other causes/problems and avoid further damages.