SQ8 Won't Start: SCR Catalyst & AdBlue System Alert

Having dealt with a similar issue on my 2018 Audi Q7 diesel, I can share my experience. The symptoms match exactly what I encountered, where dashboard alerts started with the service light before escalating to SCR system warnings. Initially, I thought it was just low DEF fluid (AdBlue), but the workshop diagnosed a faulty connection between the AdBlue tank and the dosing unit. This affected the emission standards compliance system, which triggered the starting prevention as a protective measure. The diagnostic process revealed: Tank had sufficient AdBlue, Sensor readings were inconsistent, Dosing unit was not receiving proper fluid flow The repair cost me 250 Euro, which included replacing the faulty connection components and a system reset. The workshop also performed a full diagnostic scan to ensure no other emission control components were affected. To better assist with your situation, could you provide: Current AdBlue level reading, Exact warning messages displayed, How long the service light was on before other warnings appeared, Whether you notice any unusual fuel efficiency changes This information would help determine if youre facing the same underlying issue or if its something different in the SCR catalyst system.

Thanks for the responses about the SCR system. Just wanted to update that I finally got the diagnostic results at 172342 KM. The injection system was actually working fine, but the DEF fluid sensor was sending incorrect readings to the emissions control unit. The dashboard alert showing low fluid level was wrong, tank was actually 3/4 full. They cleaned the sensor connections and reset the service light. Total repair came to 180 Euro and the car starts perfectly now.

Good to hear you got it sorted out for 180 Euro. The situation turned out quite similar to what happened with my Q7, these SCR catalyst systems can be tricky with sensor readings. My workshop also found that cleaning the connections was key, though in my case we needed a component replacement too. The service light and emissions control warnings can definitely be misleading when sensor connections get dirty or corroded, making the system think theres an AdBlue issue when the tank level is actually fine. These false readings are a common quirk with the diesel exhaust system. For future reference, its worth noting that dirty sensor connections can trigger the full range of dashboard alerts even with sufficient AdBlue levels. Regular system checks during maintenance can help catch these issues before they escalate to starting prevention. Glad you got the problem resolved at a reasonable cost. Its much better than having to replace the entire dosing unit or other major SCR system components.

Thanks for sharing the follow-up experiences. After getting the car back and driving it for a few days, I can confirm everything is working perfectly. The dashboard alert system is now showing correct DEF fluid readings and the service light has stayed off. For anyone searching with similar issues, my tank capacity is around 24 liters, and the incorrect sensor was making the system think it was empty when it actually had about 18 liters left. The fuel efficiency has remained consistent at pre-warning levels, proving it was purely a sensor communication problem rather than an actual SCR catalyst malfunction. Good to know that cleaning the connections can fix these false readings without needing expensive component replacements. The 180 Euro repair cost was definitely better than what I initially feared when seeing all those warning messages.