Multiple VW System Failures Point to Electrical Issues

I had a very similar issue with my 2008 VW Passat TDI. The symptoms you describe match exactly what I experienced, including unusual warning lights, system failures, and intermittent electrical issues. After some frustrating attempts at self-diagnosis with a basic scan tool, I took it to a workshop. The engine diagnostics revealed multiple error codes, but the root cause wasnt immediately clear. The malfunction indicator was particularly annoying, as it would out of nowhere turn on and off. The mechanic found that the cable connection to the control unit was damaged, causing poor communication with the BUS system. This explained all the unusual system failures and warning lights. The repair involved replacing the wiring harness and reconnecting the control unit properly. Total cost was 155€ and the issue never returned. To better assist with your case, could you provide: Current mileage, When these issues first started, Whether any recent repairs were done, If the problems occur more often in wet weather, Which specific error codes your scan tool shows This information would help determine if youre dealing with the same root cause or potentially something different.

Current mileage is 93131km and my last service was 2 months ago. The issue started right after a heavy rainfall last week. The scan tool shows error codes related to various control modules, including P0603 and U0101. Our mechanic did replace the battery during the last service, which makes me wonder if there could be a connection. My specific warning signals seem worse during damp weather. Engine diagnostics indicate multiple system communication errors, pointing toward a possible electrical connection issue like you mentioned. This really helps narrow down where to look, especially the wiring harness connection part. Would it make sense to first check the connections around the recently replaced battery before going deeper into the control module wiring?

Thanks for providing those details. Given that the issues started after both a battery replacement and heavy rainfall, checking the battery connections should definitely be your first step. This strongly mirrors what I experienced with my Passat, water ingress near electrical connections can trigger these exact symptoms. The error codes you mention (P0603 and U0101) are classic signs of control module communication problems, which matches the service required warnings and unusual system failures I dealt with. In my case, water had actually gotten into the main wiring harness near the battery tray area, corroding several connections. Before spending money on extensive repairs, I would recommend: 1. Inspecting all battery connections for proper tightness and corrosion 2. Checking the ground straps, loose grounds can cause multiple system malfunctions 3. Looking for any signs of water intrusion around the battery area The timing with the battery replacement is particularly interesting. When my battery was changed, a protective cover wasnt properly reseated, allowing water to reach sensitive electronics. The malfunction indicator and other warning lights started appearing in exactly the same way you describe, especially in wet conditions. Getting these electrical connection issues diagnosed early can prevent more expensive control module failures down the line. The OBD-II codes you mentioned definitely point toward electrical issues rather than mechanical problems.

Thanks for that detailed response about the battery and wiring connection check. It definitely makes sense to investigate the battery area first, especially since these issues started right after the replacement and that rainfall. I just checked the battery area and found something interesting, the protective cover is slightly misaligned and there are traces of moisture around the connections. The check engine light actually stopped flickering after I cleaned and dried the area, though some warning lights still appear occasionally. I will have the mechanic inspect the ground straps and check for any water damage in the wiring harness near the battery tray. The service required notification seems less frequent now after drying everything out, but I want to make sure we prevent any future water intrusion issues. One question before I take it in, should I wait for the next rainfall to show the mechanic exactly when and how the malfunction indicator and other issues appear, or is the current evidence enough for proper engine diagnostics?