VW Jetta TDI Battery Issues: Beyond Simple Testing

I dealt with a very similar situation on my 2009 VW Passat TDI. The symptoms were nearly identical, inconsistent battery voltage readings and dim lights despite having a new car battery installed. Based on my moderate experience with electrical issues, I first thought it was just a bad battery connection, but after thorough testing, the workshop confirmed a faulty battery sensor was causing incorrect voltage readings and affecting the charging system. The repair involved: Complete diagnostic scan of the electrical system, Testing the battery charge and alternator output, Replacing the battery sensor unit, Recalibrating the new sensor Total cost came to 315€ including parts and labor. Since the fix, the electrical system has been working perfectly with consistent voltage readings and proper battery maintenance management. To provide more specific advice for your case, could you share: When was the battery last replaced? Does the problem occur more during cold starts? Have you noticed any warning lights on the dashboard? What is the current mileage on your Jetta? The battery sensor issues can be tricky to diagnose since they often mimic other electrical problems, but proper diagnosis and replacement usually resolves these symptoms completely.

Thanks for the detailed response. My last service was 2 months ago, and current mileage is 40599km. The battery was replaced 6 months ago, and low voltage issues mainly occur during cold starts. No warning lights appear on the dashboard. Had the battery tested last week and voltage readings were within normal range when running, but significantly lower during startup. After reading about similar cases, seems the alternator could also be a factor. Will have the battery voltage monitored during a full diagnostic check to rule out any charging system problems before replacing the battery sensor. Appreciate the breakdown of your repair process and costs, helps set expectations for potential fixes.

Thanks for those extra details. Having gone through this myself, the symptoms you describe match my experience, but with one key difference, my voltage drop during cold starts was more severe. The normal battery voltage readings when running but low during startup could point to either the battery sensor or alternator. In my case, battery testing showed the charging system was fine, but the battery sensor was feeding incorrect data to the ECU, causing improper voltage regulation. Before replacing my battery sensor, the workshop performed voltage monitoring across different operating conditions. Key findings showed: Battery voltage fluctuated between 11.8V and 14.2V, Battery health testing confirmed good condition, Alternator output tested normal under load The battery sensor replacement fixed all issues, but considering your milder symptoms, suggest getting the alternator tested first. The diagnostic process helped avoid unnecessary parts replacement. Quick tip from what I learned: battery health can appear normal on basic tests while the voltage sensor still misreads the actual charge state. A full diagnostic scan while monitoring real-time battery voltage provides the most accurate assessment.

Thanks for sharing your thorough repair experience. Based on your findings, I will schedule a complete diagnostic scan next week, focusing on both the battery sensor and alternator testing. Your voltage readings are particularly helpful since mine show similar fluctuations between 11.9V and 14.1V. The cold start issues combined with normal running voltage definitely align with your case. Since my battery is relatively new and testing okay, pursuing the voltage sensor diagnosis makes sense as the next step. Will make sure they perform real-time monitoring during different operating conditions. Good point about avoiding unnecessary parts replacement. Will have them test the alternator output first, then move on to the battery sensor if needed. Expecting costs around 300-350€ based on your repair bill, which helps with budgeting. Thanks again for walking through your diagnostic process. Will update once I get the results from the full system check and voltage monitoring tests.