S5 ECU Fault Prevents Engine Start, Troubleshooting Guide

I had a similar ECU issue with my 2011 Audi S4 last winter. Having some experience with engine diagnostics and electrical problems, I first suspected a more serious ECU failure but it turned out to be much simpler. The engine surge and starting problems matched your symptoms exactly, cranking but no start, check engine light appearing intermittently. After taking it to my regular workshop, they found corroded battery terminals and loose connections were causing voltage fluctuations, confusing the ECU. The total repair cost was 155 Euro, which covered cleaning the connections, applying dielectric grease, and properly securing all battery cables. The car has been running perfectly since then. Before suggesting more specific solutions, could you share: Have you noticed any corrosion around the battery terminals? Does the dashboard light up normally when you turn the key? Has the car been sitting unused for extended periods? Any recent work done near the battery or engine bay? These details would help determine if your issue matches what I experienced or points to a different ECU issue entirely.

Last service was 2500km ago with no issues found. This really helps narrow down potential causes of my engine fault. Checking the battery terminals as suggested is a good next step, I did notice some white residue around the negative terminal last week but didnt think much of it at the time. The dash lights do seem slightly dimmer than usual when turning the key, which matches what you described about voltage issues affecting the ECU. Will get the terminals cleaned and connections checked before assuming a more serious engine software problem. Thanks for helping point me in a less expensive diagnostic direction.

Glad to hear you noticed similar symptoms to what I experienced. That white residue on the battery terminal is a clear sign of corrosion that can definitely cause ECU communication problems and engine starting issues. The dimmer dash lights are another telltale indicator of poor electrical connections affecting the fuel injection system. After my repair, I learned these issues often start small but can quickly escalate to more serious engine control problems if left unchecked. The ECU needs stable voltage to properly manage fuel injection and other critical engine functions. A quick tip from my experience: while getting the terminals cleaned, have them check all ground connections too. In my case, they found a loose ground strap that was contributing to the engine light warnings and starting problems. I bet once you get those electrical connections sorted, your S5 will fire right up. Let us know how it goes after cleaning the terminals. If the problem persists, we can explore other potential causes, but start with the simple fixes first. Remember to apply dielectric grease after cleaning to prevent future corrosion. My S4 hasnt had a single engine stalling or starting issue since addressing these electrical gremlins.

Thanks everyone for the detailed feedback. Just wanted to update that I got the battery terminals cleaned and connections tightened today. The white corrosion was indeed pretty bad on the negative terminal. Shop also found and fixed a loose ground strap exactly as suggested. Total cost was 140 Euro for cleaning, new terminal clamps and labor. The engine light is now gone and the car starts perfectly, much stronger crank than before. Dashboard lights are back to normal brightness too. Really glad I checked these electrical issues before diving into more complex engine software diagnostics. The ECU seems happy with stable voltage now. Good lesson learned about not ignoring early warning signs like corroded terminals. Will definitely keep an eye on the battery connections during regular maintenance to prevent future engine control problems. Thanks again for helping solve this without an expensive diagnostic process.