Audi SQ8 No-Start: Terminal 15 Switch Failure Fix

Having dealt with a similar issue on a 2018 Q7, I can share my experience as someone with moderate expertise in car electronics. The no-start condition turned out to be related to a faulty connection between the ignition switch and the starter solenoid. The voltage drop test performed by the workshop revealed intermittent power supply issues at Terminal 15. The main problem was traced to corroded wiring connections in the fuse box area. The repair process involved: 1. Complete diagnostic scan 2. Testing the relay switch connections 3. Cleaning and reseating all related electrical connectors 4. Replacing damaged wiring sections 5. Final testing of the ignition circuit Total repair cost was 95 Euro and took about 2 hours to complete. Since the fix, no further starting issues have occurred. To provide more specific guidance for your case, could you share: Are you getting any dash lights when turning the key? Does the starter motor make any sound? Have you noticed any electrical issues before this problem started? Were there any recent repairs or battery replacements?

Last service was at 9000 KM and error persists. Following your questions: no dash lights appear, starter stays silent, and the battery terminal connections look clean. The wiring harness near the ignition switch shows no visible damage. I replaced the battery 2 months ago with no issues until now. The complete car won't start situation started suddenly yesterday morning. Another forum member suggested checking the power supply at fuse 21, but the voltage readings there are normal.

Looking at your responses, this sounds very similar to what I encountered with my Q7. Given that you have already replaced the battery and confirmed normal voltage at fuse 21, we might be dealing with a circuit break in the ignition system rather than a simple power supply issue. When my car had this exact problem, the normal voltage readings initially misled the diagnosis. The real culprit was an intermittent voltage drop that only occurred during the actual starting attempt. A more thorough testing revealed a damaged wire in the ignition harness that wasnt visibly compromised. Few key points from my experience: 1. Despite clean battery terminals, the complete lack of dash lights suggests a major break in the main power circuit 2. The sudden onset matches what happened with my vehicle 3. Normal voltage readings at rest dont always tell the full story I would strongly recommend having a voltage drop test performed across all main ignition circuit points while attempting to start. In my case, this specific test quickly identified the failing connection that wasnt obvious during static testing. Consider checking the ground straps as well, this was actually a secondary issue discovered during my repair that contributed to the no-start condition.

Appreciate all the detailed feedback, especially about the voltage drop testing. Your Q7 symptoms do sound identical to what I'm experiencing. Going to have the shop do a complete check of the wiring harness and run that voltage drop test you mentioned. The circuit break theory makes sense given how suddenly this happened. Just booked the car in for tomorrow and specifically requested they test the ignition switch pathway and check all ground connections as you suggested. The relay switch inspection also seems like a good starting point based on your experience. Really hoping its a straightforward fix like yours was. Will update once I know what the actual issue is. The complete diagnosis approach you outlined should help avoid replacing parts unnecessarily. Thanks for explaining exactly how your similar problem was resolved, its given me a clear path forward with the workshop.