Cause
Excessive post-injection fuel quantity
Too much fuel injected late in the cycle, after the main combustion event. Post injection exists to raise exhaust temperature for a particulate filter regeneration, and it does so deliberately — the question is only whether the quantity is what was intended.
Excess post injection has two consequences and both are damaging. Exhaust temperatures climb beyond what the aftertreatment can tolerate, and the fuel that does not burn runs past the rings into the sump, diluting the oil. An oil level that rises on a diesel is this happening, and it is the clearest evidence available.
The cause is usually not the strategy but a fault feeding it. A leaking injector adds fuel nobody commanded, and an exhaust temperature sensor reading low makes the module add more than it should to reach a target it has already passed. Check injector leak-off and the temperature sensors before treating the fuelling as intentional — the module is usually asking for the right thing and receiving too much.
Typically specialist work. Needs equipment or training most home workshops do not have.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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