Two part thread:

1) Cleaning oxygen and NOx sensors is easy and so helpful. I followed the advice of this thread and bought Sodium Hydroxide. I used an -- put the lye solution in the cleaner and the sensors were clean in about five minutes each. Rinsed with distilled water, cleaned with carb cleaner, and gave it a bit of the propane torch for good measure.

2) I thought changing out the DEF Pressure Sensor (Part #059906627R) would also be super easy, given that it's mounted directly on top of the engine. I've had a series of DEF and EGR error codes, so I'm generally going through parts and maintenance on my 10-year-old TDI.

tl:dr: - TRICKY! There is a rubber/EPDM housing that the sensor fits inside of, and the housing slides over a mounting post. I cannot for the life of me figure out how to remove this cover!! I need to remove it so I can put it on the new DEF Pressure Sensor.

Anybody have any luck removing that rubber mount? Pics show old DEF Pressure Sensor and rubber mount, plus one of the new sensor (no mount).

 

So, the good news is you don't need to worry about breaking the old part when you take that cover off.

Looking over your photos, it would appear to likely be a 'friction-fit' plastic protective cover (maybe don't locate it in that high heat location right behind the turbocharger next time, LOL), and it appears to be in two separate sections...the main body cover and the piece shielding the hose hookups, etc. From you pics I can see what looks like a cut line in the first photo w/cover at the bottom of it; that's where it comes apart. You know how VW likes it's covers to 'clip' into place, so that's likely same on this item to hold it closed/in place.

 

I will say, that I wouldn't replace the differential (DPF) pressure sensor unless you have monitored the sensor output and know that it's a bad sensor issue. I've got a 2010 TDI and never had an issue with that differential pressure sensor, but the NOx sensors, O2 sensors...and EGT in the exhaust have gone out. Couldn't get Audi to swap my failed EGT sensor, which doesn't throw a code out, but constantly reads a - 277 C, whether car is cold or hot. For the longest time it didn't appear to be having any impact, but now I know each one of those EGT sensors has a specific feedback 'purpose', and so yes, my failed EGT sensor is absolutely having a negative impact on the control circuit it links to.

Definitely take a look at those EGT sensor live data values (engine control module; I look at the area for the DPF regeneration monitoring, but you can directly monitor those sensors, as well...engine off/cold engine is great because all the sensors should be reading basically the same temp value, so any outlier will be extremely obvious. Engine hot is also fine, and try both if you don't see one reading extremely hot or extremely cold compared to the other EGT sensors, etc.

Conventional wisdom states that there should be some kind of fault code if a sensor is that far off the reservation, but unfortunately they don't all throw a code when they go bad, which makes interpreting the downstream 'symptoms' they create more difficult, since you aren't sure where to look. Check your coolant temp sensors output values while you at it, as those are known to go bad too, and you don't get a direct code, but might see something linked to temperature too low, etc.

Another thing that creates unforeseen symptoms/ problems is the vacuum leak that commonly occurs (on all older Q7s) where the brake booster pipe meets the engine's vacuum line at the firewall, just inside the engine bay past firewall. The female side connector (brake booster's vacuum line) cracks due to heat cycling stress. Vacuum leak creates problems for vacuum actuated solenoids, which causes...(EGR and other problems); see where I'm going with this?