VW Passat Diesel Startup Failure: AdBlue System Alert

Had a similar issue with my 2016 VW Tiguan TDI. The injection system showed the same symptoms, engine not starting and service light coming on. Initial diagnosis revealed the SCR catalyst system had an AdBlue dosing unit malfunction, not just an empty tank. The real problem was the connection between the AdBlue tank and the dosing module. Crystallization had formed in the lines, completely blocking the flow. This affected the entire exhaust aftertreatment system and triggered the engine immobilizer as a protective measure to meet emission standards. The repair required: Complete cleaning of the AdBlue lines, replacement of the dosing module, recalibration of the injection system, tank capacity sensor replacement. This was definitely a serious repair job that took several days at the workshop. Not something that can be fixed by simply refilling the AdBlue tank. The crystallization issue is common in these models when regular maintenance intervals are missed. Would recommend finding a shop specializing in VW diesel systems, as they will have the right diagnostic tools and experience with the SCR system components.

Thanks for sharing your experience. I had a nearly identical situation with my Passat when the DEF fluid system failed. The service light came on and the engine refused to start after several attempts. My issue also turned out to be crystallization in the SCR catalyst system. Quick question, do you remember roughly how much you paid for the full repair? Did any other emissions control issues come up in the months after fixing it? Want to know if I should budget for potential follow-up problems.

Had the repair done last month, total cost was 250€ to fix the faulty AdBlue output connection between the tank and dosing unit. The mechanic confirmed it was causing the starting issue and dashboard alert. No additional emission standards problems since the repair. The SCR catalyst system now works perfectly and fuel efficiency is back to normal. The crystallization was cleaned out completely and all connections were properly sealed. Would recommend getting it fixed right away if experiencing similar symptoms. Leaving it too long risks more extensive damage to the tank capacity sensors and exhaust components. Important to catch these issues early before they cascade into bigger problems with the entire emissions control system. The repair price was reasonable considering the technical work involved with the AdBlue system. Regular maintenance helps prevent these issues from recurring. My tank levels and dosing are working correctly now with no warning lights.

After getting your feedback, I went ahead and took my Passat to a specialized shop. Turns out you were spot on, the crystallization in the diesel exhaust system was the culprit. The total repair came to 380€, a bit more than yours since they had to replace the entire dosing module. The mechanic did a thorough job cleaning the DEF fluid lines and recalibrating the system. No dashboard alerts since the fix, and the engine starts perfectly now. Fuel efficiency has actually improved slightly too. The shop showed me the crystallized parts they removed, pretty shocking how blocked up it was. They recommended checking the AdBlue levels monthly to prevent future issues. Been doing that religiously and monitoring the emissions system closely. Really glad I asked here first instead of taking it to my usual mechanic who probably would have missed the real problem. Thanks again for pointing me in the right direction with the SCR catalyst system diagnosis.