S8 Engine Stalls: Start-Stop System Failure Fix Guide

Had the same issue on a 2020 A6 with the automatic restart system. The problem turned out to be a combination of a failing starter motor and a deteriorating battery. The engine start problems began similarly, unexpected shutdowns beyond the normal idle stop function. The engine control unit stored error codes related to the starter motor circuit and battery voltage. When tested, the battery showed significant degradation affecting the hybrid vehicle systems, particularly the start-stop functionality. The repair required: Full battery replacement, New starter motor installation, Reset of the engine control module, Calibration of the start-stop system This was a serious repair since both components are critical for the Eco mode and automatic restart features. The failing battery caused additional strain on the starter motor, creating a cascade effect. After the fix, the start-stop system worked flawlessly again. Would recommend getting both components checked, as replacing just one might not solve the underlying issue. The symptoms match classic signs of a compromised starting system in modern vehicles with automatic restart technology.

Thanks for sharing your experience. I had the identical issue with my S8 last year. The automatic restart system failed the same way with unexpected engine stalling. The hybrid vehicle system diagnostics pointed to the battery, but my case turned out to be mainly an issue with corroded connections at the battery terminals rather than the battery itself. The idle stop feature started working properly after cleaning the connections and adding protective coating. The entire service including diagnostics and labor came in much cheaper than a full battery replacement. Though they did mention I should plan for a new battery within the next year. Did you track how much you ended up paying for your repair? Have you experienced any other issues with the Eco mode or start-stop system since the fix?

The total repair bill came to 290 Euro for replacing the heavily discharged battery, which ended up being the core problem. Since having the car serviced, the automatic restart system works perfectly, no more unexpected engine stalling or start-stop malfunctions. In my case, the mechanic confirmed the starter motor was actually fine after testing, despite initial concerns. The car battery was the sole culprit causing the engine start problems. The hybrid vehicle systems require a strong, reliable battery to function correctly. Good to hear you managed to fix yours with just a terminal cleaning. But even with clean connections, aging batteries can still cause start-stop system issues since they need consistent voltage to operate properly. After three months of trouble-free operation, I can confirm getting the battery replaced was the right call for my situation.

Finally got it fixed last week. The idle stop issues were getting worse, so I took it to a different mechanic who specializes in hybrid vehicle systems. Their diagnostic showed my car battery was operating at just 60% capacity, causing the automatic restart problems. The repair was straightforward: New AGM battery installation, System reset and programming, Start-stop system calibration Total cost came to 450 Euro including labor. More than I wanted to spend, but the car starts perfectly now and the idle stop feature works like new. No more warning lights or unexpected engine stalling. Worth every cent compared to the stress of unusual shutdowns. Ended up leaving a positive review for the new shop. They took time to explain everything and even showed me the battery test results. Much better experience than my previous mechanic who just kept resetting the system without fixing the root cause.