Active Regeneration Function
The soot load of the particulate filter is calculated by two pre-programmed load models in the Engine Control Module (ECM) J623.
One of the load models is determined from the driving profile of the user and the signals from the exhaust gas temperature sensors and Heated Oxygen Sensor (HO2S) G39.
Another soot load model is the flow resistance of the particulate filter. It is calculated from the signals of Exhaust Pressure Sensor 1 G450, Exhaust Gas Temperature (EGT) Sensor 3 G495, and Mass Air Flow (MAF) Sensor G70.
Obvious question would be how many of these sensors got tested before being reinstalled into the successive DFP shells (@~40k & 51k miles). History shows on DP sensor replacement at ~40k. Other sensor issues could have harmed active regeneration. No reference to replacement of the Mass Air Flow (MAF) Sensor G70. The dealer missed something during the 40k, 50k, and probably 51k services. Low MPG (higher than normal fuel consumption) suggests that combustion is off. It's not hard to imagine this leading to a significant increase in DPF loading. Incomplete combustion = more unburnt fuel = more soot creation..... Remember, exhaust temps are already being squeezed down by the use of EGR. An incorrect split of too much HP EGR v.s. LP EGR could deprive the turbo of motive power that could also impact combustion. Or perhaps the Oxidation Catalyst section died first killing active regeneration. Someone would have to autopsy the DPF's to see what happened inside. Maybe the OP had an internally cracked DPF?
Joseph - Did you notice an improvement in engine power comparing the prior failure period (20-30k miles) with power after the 51k mile DPF replacement?
There are so many interdependencies in this system that it's hard to pick a good starting point to really diagnose the problem. GFF (Guided Fault Finding) has it's limits. Reading the narrative about the two 'sub-stochiometric combustion processes associate DeNOx and DeSOx is a real head banger. In terms of controls, there are a lot of potential things that can go wrong. My bet is that the root cause will be found looking at the components that manage active regeneration.
The turbo replacement sounds like the turbine IGV's were severely coked up to a point where they wouldn't move. Turbo IGV fouling likely developed during the first DPF failure. How could the dealer replace the turbo and not realize another DPF failure would be right around the corner? Yeah - I know - GFF tells them what to do.... I suspect these days parts get thrown at problems in a very scripted manor. What ever happened to reading the story the parts tell?
It's becoming clear that combustion has been "off" in this car for an extended period. This begs the question about potential impacts to the piston ring belts (i.e. carbon fouling). This could impact piston sealing. This might be evidenced by higher oil consumption, because fouling would impact oil scraping action in the lower ring belt. Compression testing of this vehicle should be done to confirm that the rings are doing their job. Or at least the car should get a dyno run to check power delivery and combustion parameters. Probably not part of the GFF followed for this vehicle. It's also possible that some of these issues began with ring problems? The EDC-17 is supposed to tune the engine to compensate for component wear. Wonder if it would try to compensate for fouled / worn rings?
I'm not trying to confuse this issue. But in my mind there are many more questions than answers at this point. I'm having a big problem with the lower fuel mileage (high fuel consumption) of this vehicle. If that isn't fixed - The real problem isn't fixed..