VW Lupo: Multiple System Failures Point to Wiring Issue

As someone with moderate experience working on VWs, I dealt with an identical issue on my 2005 VW Polo diesel. The symptoms were exactly as you describe, unusual system failures, control module errors, and intermittent electrical issues affecting multiple components. After connecting an OBD-II scanner, the stored error codes pointed to communication problems between various control units. The root cause ended up being a damaged cable connection to the main control module where it interfaces with the vehicle BUS system. The service required was relatively straightforward once properly diagnosed. The workshop found corroded pins in the main harness connector that were causing intermittent connection losses. Total repair cost was 155€ including diagnostic time and parts. Before proceeding with repairs, it would be helpful to know: Have you noticed these issues more frequently when driving over bumpy roads? Does the check engine light stay on permanently or flash intermittently? Are the problems worse during wet weather? Can you share the specific error codes from your scan tool? This information would help confirm if you are dealing with the same underlying connection issue I experienced. These symptoms often point to wiring problems rather than failed components, which is generally good news for repair costs.

Thanks for sharing your experience. After the latest service at 85046km which included a full diagnostic scan, the engine check light started appearing more frequently, especially on rough roads. The OBD-II scan revealed similar communication errors across multiple control modules. I checked the wiring connections as suggested and found significant corrosion on the main harness connector near the engine control module. This definitely matches the symptoms you described with the intermittent system failures. My mechanic quoted 180€ for a complete harness repair including cleaning the corroded pins and replacing damaged sections. Does this pricing align with your repair costs? Should I also request testing of the ground connections while they have the harness exposed?

Thanks for the follow-up about your Lupo issue. The 180€ quote for the harness repair is slightly higher than what I paid, but still reasonable considering current parts and labor rates. The extensive engine diagnostics and testing needed for intermittent electrical faults can be time-consuming. While they have the harness exposed, absolutely get them to check and clean all ground connections. Poor grounds can cause similar warning signals and system failures. In my case, we discovered two corroded ground points that were contributing to the communication errors between control units. After my repair was completed, I made sure the workshop performed a full system reset using their professional diagnostic tool. This cleared any stored error codes that might trigger false service required messages. The unusual electrical issues completely stopped after addressing both the harness corrosion and ground connections. A key tip from my experience: make sure the repaired areas get properly sealed against moisture to prevent future corrosion. The OBD-II communication problems often return if moisture protection is overlooked during reassembly. Let us know how the repair turns out. These electrical gremlins can be frustrating but are usually fixable once the root cause is properly identified.

Thanks for the detailed responses. I went ahead with the full harness repair and ground connection service. Total cost came to 195€ which included extra waterproofing of all connection points and a complete system reset using their scan tool. The warning light issues have completely stopped after 2 weeks of driving. All control units are now working perfectly and no more unusual system failures occur even on rough roads. The check engine light has stayed off and the latest OBD-II scan shows no stored error codes. My advice for others with similar symptoms: have your mechanic thoroughly check all main harness connections and ground points before replacing expensive control modules. The service required ended up being much simpler and cheaper than initially feared. Getting a proper diagnostic scan was key to identifying the actual problem rather than just guessing at failed components. Really appreciate the guidance that helped pinpoint the root cause. Would have spent a lot more time and money troubleshooting without the specific advice about checking those harness connectors.