Starting in August, I noticed the Q7 seemed to be consuming a lot of DEF. It also had a fault (but no CEL):38161 - Reductant Metering Module
Brought it in to Audi, and they replaced the rear NOX sensor. I drove the car about a hundred miles, then took it to CT emission testing, where it failed for readiness (I didn't know that you have to drive a TDI to the moon and back to set readiness).
So after failing I kept driving it, now periodically checking to see if it passed readiness. It didn't. Instead, it threw a new fault, and eventually the CEL came on.
Back to the dealer it goes. Anyone want to guess what the fix is? 17277 - SCR NOx Catalyst Bank 1

P20EE 00 [10101111] - Efficiency too Low

MIL ON - Confirmed - Tested Since Memory Clear

Freeze Frame:

Fault Priority: 2

Fault Frequency: 2

Mileage: 181140 km

Date: 2025.03.09

Time: 16:54:12



Engine RPM: 1861.50 /min

Normed load value: 52.5 %

Vehicle speed: 23 km/h

Coolant temperature: 92 °C

Intake air temperature: 17 °C

Ambient air pressure: 1000 mbar

9:35 PM 3/15/2025 Voltage terminal 30: 14.360 V

Unlearning counter according OBD: 40

SCR efficiency test overall: 27.20 %

SCR error threshold of efficiency test: 45.00 %

SCR efficiency test 2: 0.320

SCR efficiency test 1: 0.249

Particle filter: unburned fuel calculated: 0.004

Mean temperature of SCR catalyst: 245.1 °C

Long-term adaptation of reduction agent dosage: 0.92





Readiness: 1 1 0 0 0

 

I'm gonna guess the upstream (#1) NOx sensor, but I'd also look at the live data on the gaggle of EGT sensors in the exhaust to see if there's any outliers there as those don't always throw a code. Check the coolant temp sensors too, just for grins.

When my reductant system was showing me crazy no-restart messages with the mileage value jumping around, and car was in-warranty still, Audi stealer just started replacing parts on it...that didn't fix it. Then they swapped one or both the NOx sensors, which is what fixed the underlying issue. Never could get them to replace my EGT sensor pre-DPF as there was no code or perceived issue linked to that EGT sensor reading a constant value (-273 C).

FWIW: Ran across a thread on Ross-Tech Wiki looking up your fault code, and I'll post you the link to that. Long story short, you can clean and reuse those expensive NOx sensors, if you know what to use on them and are so inclined to save $$$. Same with those O2 sensors too, normally. Read all about it here:

https://forums.ross-tech.com/index.php?threads/5878/

 

Thanks for the link. I may fool around with VCDS to see what I can learn by looking at EGTs.
I should have mentioned that I am one of the very few who are still in warranty. Still have 7k miles or 5 months. ;-)

 

LOL, I figured you still had TDI extension time remaining or aftermarket service contract as you weren't grousing about how much they Audi Stealer charges for those friggin NOx sensors + diagnostic + installation, etc. Those little buggers are in the category of most expensive sensor to have to buy, so it's nice to know they can be cleaned and reused. I wasn't aware of that either, so thanks for posting your issue! That'll save me some money in the future, no doubt about it.

 

So I have the Q back after three days at the Audi spa.
Short story is that no parts were replaced (which always worries me) but they did do a bunch of testing, including output tests and other witchcraft that showed that the cat was healthy...which kinda makes sense given the lowish miles (112k) and the condition.
I'm out of pocket for a bit, but will drive the crap out of the thing until it either passes readiness or dies trying.

Relatedly, I need to get this done by May 15 when my registration expires, because Connecticut won't renew it without a happy test result.

 

Yeah, yet another shining example of Audi Tech/Service Dept. superiority.

You give them the car for three days with a known, continuing issue, and at the same dealership that's been unsuccessfully troubleshooting said issue and they give the car back to you saying "everything we checked looks good".

I'd put the VCDS on it and do some logging of both the NOx and EGT sensors, etc. while driving. It seems fairly logical to me, a self-taught shade-tree mechanic, that when you've replaced everything else within the emissions system contol loop except for one emissions sensor that sets the baseline NOx reading, then it's pretty clearly the one sensor you haven't changed yet. LOL! Now if you were out of warranty they'd be telling you you need the control module/ECU replaced, and I'm not even kidding.

 

750 miles since I picked the car up from the dealer, and I passed CT emissions today with seven out of eight readiness codes set.
At least the monkey is off my back for now, but I can't help but wonder:
Will the last readiness code set?
When?
Will the CEL return first?
No fault code or pending codes as of now.

More to come...

 

1200 miles and still waiting for the catalyst efficiency readiness code to set. Crazy.

 

Diesel catalyst is the SCR catalyst, post-DPF, where the Adblue (Urea) injector sprays into the exhaust flow post DPF and the NOx reduction magic happens in that catalyst unit. The control mechanism for the SCR catalyst includes the NOx sensors upstream (not replaced) and downstream (replaced), as well as EGT sensor feedback.

I'd do quick verify cold of all the EGT sensors (Engine - Advanced Measuring - type in temperature sensor in search box...select all the EGT sensors) looking for outliers. Same with NOx sensors to baseline...both should be at zero when cold/engine not running. If no outliers seen, repeat same with car at normal temp.

I'm pretty certain that's where you'll find the real issue...one of those sensors is flaking on you. Problem is they don't throw fault codes unless they are shorted out internally. That's actually how I got a whole new SCR system on my TDI...typical Audi troubleshooting...replace a part, drive test/fail...replace a part, scan/drive-test/fail, etc. So much for GFF troubleshooting they use at dealerships, but I can't complain! It was only after they'd replaced every single thing in the SCR that they got around to verifying my NOx sensors, and that was the underlying issue.

I've got a bad EGT for a year now in the pre-DPF location...absolutely nothing from the car about it...still regenerates DPF. I disconnect the faulty sensor and the car throws a code, reconnect it and the car is 'happy' again, but the sensor is reading - 277C in one view, and in another related to the DPF it reads a constant state of 324C, IIRC. Definitely bad EGT sensor, and I'd think the car would notice that, but no outward warnings or indication. I'll have to check my SCR readiness indicators too, just for grins; they don't check the emissions where I live, so hadn't thought to look at that dataset.

 

Thanks. I'll give this a shot.