This is a fun one.
I've had a 2005.5 Audi S4 Manual 6spd for about 11 years. It has an electrical gremlin where it literally cuts out for a split second where the CEL and EPC light would pop on as well before disappearing. I've had it this problem for almost as long as I've owned the car but never could identify the source.
The difficulty in doing so lies in the fact that one, it would happen very sporadically, once every few months, weeks, days... and two, it would happen while cruising, part throttle tip-in, and/or at WOT. The biggest factor is that it would completely reset the ecu.
This was indicated by zero'd out fuel trims and any current DTCs being erased. Readily replicating the problem is nearly impossible but it is more susceptible at WOT/higher load than other conditions. Vehicle is now turbo but occurred even when NA.
Over the last year or two it's been getting WORSE. The ECU would even cut off at idle on a few occasions resulting in the engine shutting off of course. If I'm driving down the road, then it feels like a big misfire but since I'm in gear, the engine keeps going.
Diagnostic steps include:
Swapping ECU (went from B6 to B7 ECU, granted this was more so for ease/speed of flashing via NefMoto software)
Swapping ECU main power relay
Swapping Fuel Pump relay
Wire harness removal, inspection, and tone generator wire tracing (in progress as of last night 09DEC2024)
There is one code I've had pop up for many years and that is a low voltage code (terminal 30) which is strange because that's constant 12v. I never found any issues.
Earlier this year when one of the resets occurred, the ECU finally stored five codes:
Short to grounds:

I've had a 2005.5 Audi S4 Manual 6spd for about 11 years. It has an electrical gremlin where it literally cuts out for a split second where the CEL and EPC light would pop on as well before disappearing. I've had it this problem for almost as long as I've owned the car but never could identify the source.
The difficulty in doing so lies in the fact that one, it would happen very sporadically, once every few months, weeks, days... and two, it would happen while cruising, part throttle tip-in, and/or at WOT. The biggest factor is that it would completely reset the ecu.
This was indicated by zero'd out fuel trims and any current DTCs being erased. Readily replicating the problem is nearly impossible but it is more susceptible at WOT/higher load than other conditions. Vehicle is now turbo but occurred even when NA.
Over the last year or two it's been getting WORSE. The ECU would even cut off at idle on a few occasions resulting in the engine shutting off of course. If I'm driving down the road, then it feels like a big misfire but since I'm in gear, the engine keeps going.
Diagnostic steps include:
Swapping ECU (went from B6 to B7 ECU, granted this was more so for ease/speed of flashing via NefMoto software)
Swapping ECU main power relay
Swapping Fuel Pump relay
Wire harness removal, inspection, and tone generator wire tracing (in progress as of last night 09DEC2024)
There is one code I've had pop up for many years and that is a low voltage code (terminal 30) which is strange because that's constant 12v. I never found any issues.
Earlier this year when one of the resets occurred, the ECU finally stored five codes:
Short to grounds:
Inj 1, 4, 8
Evap Solenoid (N80)
Internal RAM error
Wiring diagrams show shared 12v amongst those components however, that's switched 12v (terminal 15) things. I'm not certain that would cause a hard ECU reset like what disconnecting the battery would do, but I could be wrong so here at wit's end looking for experts.
Note: This ECU does not give freeze frame data which also sucks.
Code:
Chassis Type: 8E (8E - Audi A4/S4/RS4 B6/B7 (2001 > 2008))
Scan: 01 02 03 08 09 0F 11 15 16 17 18 25 36 37 45 46 55 56 57 65
67 69 75 76 77
VIN: WAUZZZ8E14A070783 Mileage: 189190km-117557miles
-------------------------------------------------------------------------------
Address 01: Engine Labels: 079-910-560-BBK.lbl
Part No SW: 8E0 910 560 G HW: 8E0 907 560
Component: 4.2L V8/5V G 0040
Revision: 00000000 Serial number: AUZ6Z0C0242432
Coding: 0011711
Shop #: WSC 87119 806 641192
VCID: 295922AA83FAA1C5F5-5124
VINID: 9D196AAAA2150444401C01C80C63F280E1
16666 Fuel Injector #8 (N86)
P0282 002 Short to Ground Intermittent
17833 EVAP Purge Valve (N80)
P1425 002 Short to Ground Intermittent
16645 Fuel Injector #1 (N30)
P0261 002 Short to Ground Intermittent
16654 Fuel Injector #4 (N33)
P0270 002 Short to Ground Intermittent
16988 Internal Control Module
P0604 008 RAM Error
Readiness: 0110 0100
-------------------------------------------------------------------------------
Address 03: ABS Brakes Labels: 8E0-910-517.clb
Part No SW: 8E0 910 517 C HW: 8E0 614 517 BB
Component: ESP8 quattro H04 0180
Revision: 00000000 Serial number: 00000000000000
Coding: 0004901
Shop #: WSC 06335 000 00000
VCID: 275D249296EE97B5E3-8072
1 Fault Found:
01826 - Sensor for Steering Angle (G85); Supply Voltage Terminal 30
000 - -
-------------------------------------------------------------------------------
Address 08: Auto HVAC Labels: 8E0-820-043.lbl
Part No: 8E0 820 043 AM
Component: A4 Klimaautomat 2731
Coding: 00000
Shop #: WSC 00000
VCID: 6CD71BBE1C8874ED4E-5124
1 Fault Found:
00819 - High Pressure Sensor (G65)
07-00 - Signal too Low
-------------------------------------------------------------------------------
Address 09: Cent. Elect. Labels: 8E0-907-279-8EC.lbl
Part No: 8E0 907 279 J
Component: int. Lastmodul USA 0503
Coding: 01023
Shop #: WSC 65536
VCID: 3979F2EA235A114585-4B00
4 Faults Found:
01496 - Bulb for Fog-Lamps; Left (L22)
30-00 - Open or Short to Plus
01500 - Bulb for Fog-Lamps; Right (L23)
30-00 - Open or Short to Plus
01518 - Bulb for Back-Up Lights; Left (M16)
30-00 - Open or Short to Plus
01519 - Bulb for Back-Up Lights; Right (M17)
30-00 - Open or Short to Plus
-------------------------------------------------------------------------------
Address 0F: Digital Radio Labels: 8E0-035-593-XM.lbl
Part No SW: 8E0 035 593 E HW:
Component: SDAR XM
Revision: Serial number: AUZ4Z7D3021736
Shop #: WSC 00000 000 00000
VCID: 2B55D8A28906B3D507-515E
1 Fault Found:
02635 - Tuner Not Enabled/Activated
000 - -
-------------------------------------------------------------------------------
Address 15: Airbags Labels: 8E0-959-655-94.lbl
Part No SW: 8E0 959 655 H HW: 8E0 959 655 H
Component: Airbag 9.41 H10 3230
Revision: 91H10323 Serial number: 0036N000CWJ9
Coding: 0032707
Shop #: WSC 06335 000 00000
VCID: 3C77EBFE7F68246D9E-8068
Subsystem 1 - Part No: 8E0 910 339 B
Component: BF-Gewichtsens. 02 0070
Serial number: 6332DRB401818618G
Coding: 00032707
Subsystem 2 - Serial number: 6342DRB401829425F
Subsystem 3 - Serial number: 6352ARB4003B259CO
Subsystem 4 - Serial number: 6362ARB4003B62308
Subsystem 5 - Serial number: 63729RB4005238089
Subsystem 6 - Serial number: 63829RB400523AC9P
1 Fault Found:
01316 - ABS Control Module
004 - No Signal/Communication - Intermittent
-------------------------------------------------------------------------------
Address 16: Steering wheel Labels: 8E0-953-549.lbl
Part No: 8E0 953 549 Q
Component: Lenksáulenmodul 0601
Coding: 01041
Shop #: WSC 06335
VCID: 41899A0A4B8A4985CD-4B00
No fault code found.
-------------------------------------------------------------------------------
Address 17: Instruments Labels: 8E0-920-9xx-8EC.clb
Part No SW: 8E0 920 981 Q HW: 8E0 920 981 Q
Component: KOMBI+WFS 4 H12 0040
Revision: 0040 Serial number: 00000000000000
Coding: 0023204
Shop #: WSC 02301 444 10073
VCID: 3B75E8E27966235597-5124
1 Fault Found:
01304 - Radio
004 - No Signal/Communication - Intermittent
-------------------------------------------------------------------------------
Address 25: Immobilizer Labels: 8E0-920-9xx-8EC.clb
Part No SW: 8E0 920 981 Q HW: 8E0 920 981 Q
Component: KOMBI+WFS 4 H12 0040
Revision: 0040 Serial number: 00000000000000
Coding: 0023204
Shop #: WSC 02301 444 10073
VCID: 3B75E8E27966235597-5124
1 Fault Found:
01304 - Radio
004 - No Signal/Communication - Intermittent
-------------------------------------------------------------------------------
Address 36: Seat Mem. Drvr Labels: 8E0-959-760.lbl
Part No: 8E0 959 760
Component: Sitzmemory R2 F 0305
Coding: 00002
Shop #: WSC 00000
VCID: 71E90ACA3BAA99057D-4ACE
1 Fault Found:
00668 - Supply Voltage Terminal 30
07-10 - Signal too Low - Intermittent
-------------------------------------------------------------------------------
Address 37: Navigation Labels: 8E0-035-192-RNSE.clb
Part No SW: 8E0 035 192 E HW: 8E0 035 192 E
Component: RNS-E US H54 0260
Revision: 03S Serial number: AUZBZ7E1501313
Coding: 0309167
Shop #: WSC 06335 000 00000
VCID: 2643299692E48EBDD8-515E
3 Faults Found:
00858 - Connection; Radio Intermediate Frequency (IF) Output to Antenna Amplifier
011 - Open Circuit - Intermittent
00466 - Control Module for Steering Column Electronics (J527)
008 - Implausible Signal
01044 - Control Module Incorrectly Coded
000 - -
-------------------------------------------------------------------------------
Address 46: Central Conv. Labels:. 8E0-959-433-MAX.clb
Part No: 8E0 959 433 BT
Component: Komfortgerát T7B 3232
Coding: 11902
Shop #: WSC 06335
VCID: 0407531E14189CADA6-4AE8
Subsystem 1 - Part No: 8E1959801F
Component: Tõrsteuer.FS BRM 1315
Subsystem 2 - Part No: 8E1959802E
Component: Tõrsteuer.BF BRM 0205
Subsystem 3 - Part No: 8E0959801A
Component: Tõrsteuer.HL BRM 0205
Subsystem 4 - Part No: 8E0959802A
Component: Tõrsteuer.HR BRM 0205
2 Faults Found:
01134 - Alarm Horn (H12)
29-00 - Short to Ground
00955 - Key 1
35-00 - -
-------------------------------------------------------------------------------
Address 55: Headlight Range Labels: 4F0-910-357.lbl
Part No SW: 4F0 910 357 E HW: 4F0 907 357 E
Component: AFS 1 H01 0010
Revision: 00000001 Serial number:
Coding: 0000019
Shop #: WSC 01041 444 52566
VCID: 285F27AE9BF098CDEA-807C
Subsystem 1 - Part No: 8E0 941 329 A
Component: AFS-Modul links H05 0030
Subsystem 2 - Part No: 8E0 941 329 A
Component: AFS-Modul rechtsH05 0030
1 Fault Found:
01314 - Engine Control Module
004 - No Signal/Communication - Intermittent
-------------------------------------------------------------------------------
Address 56: Radio Labels: 8E0-035-192-RNSE.clb
Part No SW: 8E0 035 192 E HW: 8E0 035 192 E
Component: RNS-E US H54 0260
Revision: 03S Serial number: AUZBZ7E1501313
Coding: 0309167
Shop #: WSC 06335 000 00000
VCID: 2643299692E48EBDD8-515E
3 Faults Found:
00858 - Connection; Radio Intermediate Frequency (IF) Output to Antenna Amplifier
011 - Open Circuit - Intermittent
00466 - Control Module for Steering Column Electronics (J527)
008 - Implausible Signal
01044 - Control Module Incorrectly Coded
000 - -
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