On 22/12/2020 22:57, Anthony Nonymous wrote:
Hello All. Advice appreciated and I would have posted also to
uk.rec.cars.vw.watercooled, but that group seems to be pretty dead,
now.

The problem: diesel particulate filter (DPF) light came on, briefly,
about 2 weeks back. After a week or so of no problems, DPF, engine
management and curly symbol light (glow plug indicator, but also
indicating other issues) lights all came on and stayed on, with the
car stuck in limp mode. Various amateurs plugged in assorted gizmos
and failed to identify a relevant fault code. Handed over to a
trusted local garage (independent) and their diagnostics also said
all was well, so they reset the fault codes and attempted a forced
DPF regeneration; no dice. Concluded that the fault might be any
number of sensors, but that the investigative work would probably
cost a significant portion of the car's value. So, we're stuck with
car in limp mode.

One possibility for a faulty sensor (and by far the easiest to
replace) is the pressure sensor for the DPF. The only issue is, a new
one needs to be "adapted" to the car, by instruction from a gizmo.
I'm not sure whether any of the cheapo gizmos (e.g. Carista dongle
and app) will do the job and I'm not fancying paying for the VCDS
software that VW use. On the off-chance that anyone knows, advice
would be greatly received.

Apparently (according one of the amateurs and the garage), the DPF
sensor is giving sensible values for the differential pressure either
side of the DPF (what it measures to determine DPF clogging), so
replacing that sensor is perhaps clutching at straws. Then again, no
sensors were showing as faulty in the diagnostic tests run to date,
so DPF sensor seems as good a place as any to start.

A question now more for academic interest than anything: how does the
car estimate separate figures for soot and ash within the DPF if only
differential pressure is recorded? Are other sensors involved? I've
not come across discussion of them in documents returned in Google
searches. It is perhaps of relevance that the soot figures on the
limping golf showed a (clearly incorrect) large minus value on a
"Carly" OBD2 data reader. If the combined feed of data from the DPF
differential pressure sensor and A.N.Other sensor are processed to
calculate the soot value, then presumably, we have to suspect
A.N.Other sensor?

All I can suggest is get an OBD reader and provide the fault codes. If
the ECU has gone into limp mode I don't believe there aren't any fault
codes.