The dreaded engine limp mode

I've had my RS6 for some 3 years. Put up 20,000 km in mainly lond distance driving in that time despite pandemic travel restrictions. Totally trouble free with two exceptions. In Nov 2020 I overtook a multi trailer truck and pulled back in and re-engaged cruise. EPC and ABS light came on, engine lost all power and went into limp mode. On coming to a stop, the engine was ideling at above 1000 RPM and seemed quite noisy but not mechanical noise. Re-started engine and all warning lights were out and car was totally normal in sound and performance. I did not however use cruise again that day. Checked faults at first stop and none registered. The car then performed totally normally with a lot of brisk accelleration activity in exactly the same scenerio for 15 months and 14,000 Km.

Earlier this week in exactly the same situation immediately after overtaking a multi trailer truck, the ESP light came on and the engine went into limp mode. As I was at speed and the truck was close I managed to pull over before killing the engine, I only saw the ESP light but others may have come on. The first time I ideled for some time trying to assess the problem, this time it was an immediate shut down as I was aware that it worked on the previous occasion. Same outcome, re-start and it ran perfectly. Completed the days journey without cruise, maybe another 400 Km. Again no fault codes.

I have read up on many posts about this general sort of problem but none seem to have not had a fault code or a frequent re-occurance of the problem. The actual symptoms were identical in overtaking at speed, re-engage cruise, engine immediately goes into limp mode with no power. The differences were EPC and ABS lights first time and possibly only ESP light second time. Currently the car is running like a train, you wouldn't know that some fault is lurking ready to pop out at some time in the future.

With very little to go on and no codes, has anyone heard of similar? My RS4 gets equal treatment and has never thrown a light in nearly 9 years.

What tool are you using to scan for engine codes? If there are no codes it sounds like either and issue with a major connection, possibly at the ECU or maybe the ECU itself is the fault. What kind of work have you had done on the car/motor? Do you know what the previous owner has had done?

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Originally Posted by hahnmgh63

What tool are you using to scan for engine codes? If there are no codes it sounds like either and issue with a major connection, possibly at the ECU or maybe the ECU itself is the fault. What kind of work have you had done on the car/motor? Do you know what the previous owner has had done?

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RossTech / VCDS. I get the full log and the only fault is the Alarm horn which I removed some years ago. Once restarted, runs and performs absolutely normally. Full power and no lights (and no DTC). Replaced MAF's 2 years ago. 3 inch turbo back Miltek, Wagner IC, small map adjustments to cater for exh & IC. I run a P3 gauge and boost and vacuum readings are good all the time. I use VCDS regularly and not getting anything out of the ordinary. Fuel use is consistant with application.

Wouldn't be an issue if it happened all the time, over a year and 14,000 Km between occurances is a very random issue for something like an ECU issue. I lean towards the cruise control being part of the puzzle and just maybe I hit set and the brake as I reactivated cruise on this occasion but I really have no idea based on evidence. While the fault lights were different, the actual experience was identical in activation and resolution.

I have an RS plus map which I am toying with getting modified to cater for the small mods and upload that. The current map was done in UK and I have no real reason to suspect it based on the normal performance.

Definitely a tune issue.
Possibly hitting torque monitoring thresholds. "No DTC" is a nice indication of that.

I may have been obtuse. The "no DTC" just referred to no fault codes registered at that time. I have had the usual minor component issues come up in my ownership and at the time they registeded a fault, which was then fixed. Happy to look further into the torque monitoring thresholds and that could tie in with the ESP light. Clearly, a new tune from scratch is one approach that would clarify some issues. I would have started that process if I wanted any more power but I'm happy with the standard power level and can fall back on the manual RS4 if I want more power.

That's exactly what I meant.
Hitting L2 monitoring thresholds makes the ECU either reboot or go limp mode with no power (EPC, but no DTC).

ESP might indicate some load measuring issues (MAF related).
Read more at https://s4wiki.com/wiki/Tuning#Torque_monitoring if you're doing it yourself.
It mostly speaks of L1 though, L2 is a next level and it looks like you're hitting exactly that.

I'm not trying to sell you a retune. :)
If you're fine with stock power, might want to just flash back to stock.

Thanks, I'm always ready to learn so have no concern about selling me a retune, it may be the answer after all. I'm mechanical (clanky) with a reasonable understanding of basic computer software and how system control software applications work but I'm not an ECU / TCU tuner and have no upload / download equipment, only diagnostic. So having got my significant limitations on the table, I'm more than prepared to listen to anything you put my way.

You can be assured that I'm not looking at any DIY coding (bricking an RS6 ECU where I am is not an option) so I will go and read up so that I can listen with a bit more knowledge. I can see that with the physical mods, an obvious outcome should be a torque increase and I can say that early in the life of this car it had 109 software uploads showing in the register so someone was having real issues or it was a tune development mule at one point. The PO was having all sorts of issues with the tune when he bought the car and so the current tune is a modified standard tune done from UK for the PO however it is said that the only change is not monitoring the second O2 sensors (post CAT O2 sensors). Who knows? It drives really well and there have been no issues other than these two Limp mode instances. It gets more than a bit of full or heavy throttle use on the road for overtaking and most days would see 100 mph at some point so it isn't pusseying around surburbia. All this normally without an issue.

Thought I would throw in a log from today. Wouldn't you know it, the EGT sensor wasn't there yesterday. Otherwise, the only fault registering is the missing alarm horn. I have never had a transmission fault and the ABS has shown this
18256 - Please check DTC Memory of Engine Controller
P1848 - 35-10 - - Intermittent

which then vanishes. Took the car out for a run before the scan and it went well.

Tuesday,01,March,2022,16:11:59:65099
VCDS -- Windows Based VAG/VAS Emulator Running on Windows 10 x64
VCDS Version: 21.9.0.2 (x64)
Data version: 20210903 DS330.0
www.Ross-Tech.com
VIN: WUAZZZ4B94N9xxxxx License Plate:
Mileage: 128010km-79541mi Repair Order: 01 Mar 2022
--------------------------------------------------------------------------------

Chassis Type: 4B (4B - Audi A6/S6/RS6/allroad C5 (1998 > 2005))
Scan: 01 02 03 06 08 15 16 17 18 22 34 35 36 37 45 55 56 57 65 67
75 76 77
-------------------------------------------------------------------------------
Address 01: Engine Labels: 077-907-558-BCY.lbl
Part No: 4D1 907 558 B
Component: 4.2l V8/5VT *G 0002
Coding: 0005752
Shop #: WSC 87119 806 641192
VCID: 2D510670B7EF75E1DA-5122
VINID: 9E546AAAA1122447890000840CD8C77647

1 Fault Found:
17861 - Exhaust Gas Temp Sensor 1 (G235)
P1453 - 001 - Open or Short to Plus - Intermittent
Readiness: 0000 0000

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Address 02: Auto Trans Labels: 01L-927-156.lbl
Part No: 4B0 927 156 FB
Component: AG5 01L 4.2l5VT RdW 1315
Coding: 00001
Shop #: WSC 00000
VCID: 6CD7CB741C75BCE98D-5122

No fault code found.

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Address 03: ABS Brakes Labels: 4B0-614-517.lbl
Part No: 4B0 614 517 AD
Component: ESP 5.7 RS 6 2310
Coding: 06697
Shop #: WSC 00000
VCID: 63C5A048F1ABF3913C-5104

No fault code found.

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Address 35: Centr. Locks Labels:. 4B0-962-258.clb
Part No: 4B0 962 258 N
Component: Zentralverrieg.,DWA D38
Coding: 05868
Shop #: WSC 20110
VCID: 3467131404C5242995-4EE8

1 Fault Found:
01134 - Alarm Horn (H12)
49-00 - No Communications
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End-------------------------(Elapsed Time: 05:24)--------------------------

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