So many people are in denial over these things, LOL.
Here's how it goes. The DPF cracks, internally, meaning the substrate INSIDE of the housing is physically broken. When this happens, soot (the "black" visible stuff that diesels can emit) goes right past this internal regenerative filter, and continues all the way out the tail pipe. Normally, the post-DPF exhaust is completely clean, completely devoid of any soot. The inside of the pipe will be as clean as it can possibly be, it'll still look like new steel inside. An exhaust temp sensor, or lambda sensor, or a NOx sensor (where equipped... the CJAA doesn't have one of those) will literally look like you just removed it from a box brand new even after being directly in the exhaust pipes. (see the pics in post #68 further down). DPFs work that well. When they work.
When they crack, and this soot gets past, not only does it coat everything downstream with a layer of jet black, it also gets into the low pressure EGR plumbing, which by design takes its feed from AFTER the DPF, because it should be clean. It then goes through a small tube, which has a filter inside of it, which is technically a spark arrestor as there could be an errant burnt bit of ash after the DPF regen process that you wouldn't want getting drawn into the air inlet of the turbo.
When the DPF cracks, the first thing that most people will notice is the MIL comes on, with a P0401 'EGR insufficient flow'. That's because that soot that has found its way past to places it was never intended to be has clogged that little EGR tube up. Then, after a while, the P2002 'DPF efficiency' comes up, because the ECU is not seeing the pressure differential change before/during/after the DPF regen process because its insides are BROKEN.
What we've found, because those like me who see this constantly, is that there is no need to go pulling that EGR tube off to check for soot... because guess where else there'll be soot that there shouldn't be? The TAIL PIPES. Easy peasy diagnosis.
And FWIW, I thoroughly clean the tail pipes' insides after a new DPF install and test drive, so that the next time I'll know...and I have had a few that have had another DPF needed. You drive them enough, you'll need more than one. This is why deletes get so popular.