Q8 Diesel Start Failure Linked to Empty AdBlue Tank

Had this exact emissions control issue with my 2018 A7 3.0 TDI. The DEF fluid sensor was misreading fluid levels despite having adequate AdBlue in the tank. The diesel exhaust system showed multiple fault codes. Main problems found during diagnosis: Dosing valve blocked with crystallized DEF fluid, Wiring harness between tank sensor and dosing unit corroded, Supply line partially clogged The repair involved: Cleaning the complete DEF delivery system, Replacing the dosing module, Installing new wiring for the tank sensor, Full system reset of the SCR catalyst controls This was a serious repair requiring specialized diagnostic equipment. The car needs to detect proper DEF fluid flow or it will prevent starting to protect the emissions system. Without repair, the vehicle enters a reduced power mode and eventually won't start. Tank capacity returned to normal after repair, and fuel efficiency remained unaffected. The system has worked flawlessly for 40000 km since the fix. Before visiting the mechanic, request a full diagnostic scan focused on the SCR system to identify the exact fault codes. This helps narrow down whether its a sensor issue or actual system failure.

Thanks for sharing your experience with the A7. I had almost the same issue with my Q8 last year, complete DEF system failure that left me stranded. The service light came on first, then gradually got worse until the car refused to start. My service center also found crystallized AdBlue causing blockages and had to do a thorough system clean plus component replacement. The emissions system is really sensitive on these cars. Did you remember roughly what you paid for the full repair? Just wondering if costs have gone up since then. Also, has everything stayed working properly with the diesel exhaust system since the fix?

Thanks for asking about the repair costs. Just got my Q8 back from the mechanic last week after fixing that AdBlue tank issue. The final bill came to 250 Euro which covered fixing the faulty output line between the tank and dosing unit. The service light is now off and the SCR catalyst system is working perfectly. The emissions control warnings have completely disappeared, and tank capacity readings are now accurate. Definitely worth getting it fixed properly rather than trying temporary solutions. Since the repair, the diesel exhaust system has been running smoothly with no warning messages. The mechanic mentioned these issues are quite common when the AdBlue delivery system gets blocked or develops leaks. Regular checks of the emission standards compliance helps catch problems early before they lead to complete system shutdown.

Following up on my Q8 repair saga, turns out the total damage came to 850 Euro. The injection system needed more work than initially expected. The mechanic found crystallized DEF fluid had caused major blockages in multiple areas. The dashboard alert finally cleared after they: Replaced the complete dosing module, Cleaned all supply lines, Updated the control software, Refilled with fresh DEF fluid The fuel efficiency actually improved slightly after the fix. Been driving for about 2 weeks now with no emission standards warnings or problems. Learned my lesson about keeping the DEF fluid levels topped up and getting warning lights checked early. Much better experience than my last repair where they only did a partial fix. This shop took time to properly diagnose everything and showed me the damaged parts. Going back to them for future maintenance, worth paying a bit more for thorough work that actually solves the problem.