VW Phaeton Won't Start: Terminal 15 Ignition Issue

Had a similar situation with my VW Touareg 3.0 TDI from 2012, sounds exactly like your car won't start symptoms. With some experience working on VWs, I can share what worked for me. After multiple attempts to diagnose, the issue turned out to be a faulty connection between the ignition switch wiring harness and the main relay switch. The voltage drop across the connection was causing intermittent power loss to the ignition circuit. My workshop found corroded terminals in the main wiring harness connector near the fuse box under the dashboard. They cleaned the connections, replaced the damaged section of the harness, and that solved the problem. Total repair cost was 95€ including parts and labor. Before you head to a workshop, could you share: Are there any other electrical issues (dashboard lights, power windows)? Does the problem occur more often when the engine is hot or cold? Have you checked the starter relay in the main fuse box? What specific error codes are you getting? This information would help narrow down if youre dealing with the same ignition circuit issue I had.

Thanks for sharing those details. Had my car serviced last month at 12626km. The starter relay suggestion caught my attention. After further testing, noticed the voltage drop is significant when the key is turned. The starter solenoid clicks but power delivery seems inconsistent. The error codes mainly point to the ignition circuit, P0686 and P0685. Battery terminals are clean and tight, but I suspect the issue might be deeper in the wiring harness between the ignition switch and starter circuit. Would checking the voltage at different points along the wiring harness help identify where exactly the circuit breaks? Considering testing this before bringing it to a workshop.

Having gone through this exact issue on my Touareg, checking voltage points along the harness is definitely a smart approach. The clicking starter solenoid combined with those error codes strongly suggests a power delivery problem in the ignition circuit. Let me share what I learned from my repair: The voltage drop during key turn was actually traced to a degraded connection between the starter solenoid and the relay switch. Initially, we also thought it was just the wiring harness, but the real culprit was hiding in the fuse box area. For testing, focus on these points: Measure voltage at the starter solenoid connection points, Check continuity between the ignition switch and main relay switch, Test the power feed at the fuse box terminals connected to the ignition circuit In my case, the voltage readings showed normal 12V at the battery terminal but dropped to under 9V at the starter circuit when cranking. The workshop found that besides the wiring harness issue, the relay switch itself had failing internal contacts. If your voltage readings show similar patterns, you might be dealing with the same combination of issues. The repair will likely involve more than just cleaning connections.

Your car won't start symptoms combined with those specific error codes have helped narrow things down. The voltage drop during ignition and solenoid clicking match what my diagnostic results showed earlier. After testing the battery terminals again, I used a multimeter to trace voltage at key points along the ignition circuit. Found the starter solenoid is getting inconsistent power, readings drop from 12.6V to 8.4V when turning the key. The relay switch and fuse box connections all show signs of power interruption. I will check the wiring harness between the ignition switch and starter circuit next. Being able to test specific connection points before going to a workshop should make diagnosis easier. Your experience with similar voltage drops is quite helpful, will pay special attention to the relay switch connections near the fuse box since that was the culprit in your case. Just ordered a repair manual to better trace the ignition circuit paths. Will update once I identify where exactly the power delivery fails in the system. Really appreciate the detailed testing suggestions and sharing your repair experience.