Touareg No-Start Mystery: Electrical System Under Probe

Had similar symptoms with my 2018 VW Tiguan. The engine no-start combined with error codes pointed to a failing electrical connection rather than the starter solenoid. The root cause was traced to corroded wiring harness connections near the relay switch assembly. The terminals had developed resistance over time, causing voltage drop issues that prevented proper current flow to the ignition switch. The fix required: Testing the complete starter circuit, Cleaning all ground connections, Replacing damaged sections of the wiring harness, Installing new relay switches in the fuse box, Reprogramming the electronic control module This was not a quick fix, the wiring issues required careful diagnosis since damaged connections can be hard to spot. The original symptoms matched yours exactly, engine would not crank properly despite the starter solenoid testing fine. Suggest having a certified technician perform a complete electrical diagnostic. Wire harness issues often get overlooked but can cause major starting problems when left unaddressed.

Thanks for sharing your Tiguan experience. That wiring harness problem sounds exactly like what I am dealing with in my Touareg. The car won't start symptoms and power supply issues match up perfectly. I am planning to have the relay switch and battery terminals checked next week. Would you remember roughly how much the complete repair ended up costing? Also curious if you had any recurring issues with the fuse box or electrical system since getting it fixed?

Thanks for asking about the follow-up costs. After taking it back to the mechanic, they identified and fixed the Terminal 15 ignition positive interruption that was causing the starting problems. The repair was straightforward, it came down to a faulty cable connection in the power supply circuit that needed replacement. The total cost was 95€ for parts and labor, which was much less than I initially feared. Since fixing that connection in the ignition switch circuit, the car has been starting perfectly with no recurring electrical issues. The fuse box and relay switch assembly have been working as they should. This type of repair is typically less expensive than complete wiring harness replacement, especially when the issue is isolated to a specific circuit break. Getting it diagnosed and fixed early likely prevented more extensive electrical system damage.

Thanks everyone for the help. After your advice, I took my Touareg to a certified mechanic yesterday. They ran a full diagnostic and found the exact issue, a failing circuit break in the main power supply line. Some corrosion had built up around the battery terminals too. The repair only took about 2 hours. They cleaned all connections, replaced the damaged wiring section, and installed a new relay switch. Total cost was 220€, which felt fair given the work involved. Car starts perfectly now and all error codes are gone. Really glad I got a second opinion instead of replacing the whole wiring harness like the first mechanic suggested. Sometimes the simpler fix is the right one. Just wanted to close the loop and share the outcome. Will definitely keep an eye on the ignition switch and electrical connections going forward.