Audi A6 DEF System Failure Causes No-Start Issue

Had the exact same issue with my 2016 Audi A4 TDI last winter. With some experience working on diesel engines, I recognized this was likely related to the emissions control system. The dashboard alert started with a DEF warning, then progressed to engine start prevention. The problem turned out to be a faulty connection between the tank capacity sensor and the AdBlue dosing unit. The SCR catalyst system requires proper DEF fluid flow to maintain emissions control standards. Had it fixed at a workshop, total cost was 250 Euro for parts and labor. They found the output line from the tank to the dosing unit was defective and needed replacement. After the repair, fuel efficiency returned to normal and all warning lights cleared. To help diagnose your specific case, could you share: Current mileage, Full error codes from the diagnostic scan, Whether the DEF tank was recently filled, If you notice any unusual exhaust smell, Temperature conditions when the problem started This information would help determine if your issue matches what I experienced or if its potentially something different.

Last service was done at 155000km just 3 months ago. Error codes from diagnostic scan show P20EE and P204F. The DEF tank is about 25% full according to the gauge. Workshop quoted 850 Euro for diagnosis and potential SCR catalyst replacement. I hoped it could be just a sensor issue given the recent maintenance, but the injection system warning lights are concerning. Has anyone found a more cost-effective solution before committing to such an expensive repair?

Thanks for providing those details. My A4 showed similar error codes when I faced this issue. Based on the codes youre sharing (P20EE and P204F), this points to SCR catalyst efficiency below threshold, which matches what I experienced. Before spending 850 Euro on a full SCR replacement, consider these points from my experience: 1. The P204F code often indicates poor DEF dosing rather than a failed catalyst. My repair involved fixing the dosing unit connection, not replacing the entire SCR system. 2. With your DEF level at 25%, try a complete refill. Sometimes low fluid levels can cause irregular dosing and trigger these emission standards warnings. 3. Have the DEF quality tested. Contaminated or degraded fluid can cause similar injection system issues. My workshop found crystallized DEF was blocking proper flow. 4. Check all connections between the DEF tank and dosing module. The diesel exhaust system is sensitive to any leaks or loose fittings. The 250 Euro repair I mentioned earlier resolved these exact symptoms without needing the full catalyst replacement. Consider getting a second opinion focusing on the DEF delivery system before proceeding with the more expensive repair.

Thanks for the input. After reading about similar experiences, I will try getting the DEF tank completely refilled first and have the connections checked. The huge price difference between a potential dosing unit repair versus full SCR catalyst replacement makes it worth exploring these options. Will get a second opinion from another workshop, specifically asking them to inspect the DEF injection system and test the fluid quality. The fuel efficiency has been slightly worse lately, which matches what others mentioned about dosing unit problems. If anyone has recommendations for reliable diagnostic tools to read more detailed error codes from the emission standards system, please share. Will update once I know more about what fixed the issue.