Audi A4 Diesel Won't Start Due to SCR Catalyst Issue

As an Audi owner with some experience in DEF-related issues, I had a similar problem with my 2014 Audi A6 diesel. The service light and dashboard alert showed the same SCR catalyst error message. Initially, I also thought it was just low Ad-Blue fluid, but after checking the tank capacity, it turned out the injection system had a fault. The Ad-Blue output from the tank to the dosing unit was defective, preventing proper flow to the SCR system. The repair took about 6 hours at the workshop, and the total cost was 250 Euro including parts and labor. The fuel efficiency remained the same after the fix, which was a relief. To help diagnose your issue more accurately, could you share: Current mileage on your A4, When was the last Ad-Blue refill, Any other warning messages before the no-start condition, Have you had any previous DEF system repairs This information would help determine if youre facing the same issue or if its something different in the SCR system.

Last service was at 28240 KM and thats my current mileage too. I never had any DEF system repairs before. The only prior warning was a low DEF fluid warning about a week ago, but I ignored it thinking I could get it filled during my next service. The emission standards warning appeared right after, followed by the SCR catalyst error on the dashboard alert. This is helpful information about the injection system, I will have it checked. Hope the repair costs stay within the range you mentioned.

Thanks for providing those details. Based on your mileage and the sequence of warnings, this is definitely due to running the DEF tank completely empty, which triggered the emissions control system shutdown, a different scenario from my injection system fault. The low fluid warning typically appears when theres about 2.4L left in the tank capacity. Running completely empty can sometimes cause the system to need a full diagnostic reset, even after refilling, to clear the dashboard alert and service light. A repair shop will need to: 1. Fill the DEF tank 2. Clear the diesel exhaust system error codes 3. Possibly perform a SCR system reset Good news, this should cost less than my injection repair, likely around 120-150 Euro for the fluid, diagnostics, and reset. The repair time is typically under 2 hours. For future reference, its crucial to address DEF fluid warnings promptly, manufacturers designed these shutdowns deliberately to maintain emissions control compliance. Once the fluid is completely depleted, the engine wont restart until the system is serviced.

Thanks for that detailed information. I should have taken the DEF fluid warning more seriously, lesson learned about not ignoring dashboard alerts. The repair cost range you mentioned sounds much better than I expected. I just scheduled an appointment with my repair shop for tomorrow. They confirmed over the phone that your assessment matches their initial diagnosis, empty DEF tank causing the SCR system shutdown. They estimated about 140 Euro for the full service including the fluid refill and system reset. Going forward, I will make sure to maintain proper DEF fluid levels to avoid compromising the emissions control system. I appreciate the heads up about the diagnostic reset requirement, that explains why the engine refused to start even though its just a fluid issue. This was really helpful in understanding how these emission standards systems work. Will update the forum once the repair is complete in case others run into similar situations with their diesel vehicles.