SQ5 TDI Won't Start, DEF Tank Empty Warning Alert

The starting issue sounds familiar. Had this exact problem on a 2019 A6 TDI. The DEF fluid tank sensor was giving false readings despite having adequate fluid levels. The SCR catalyst system requires proper DEF fluid pressure to meet emission standards. Initial diagnosis showed tank capacity was fine, but the dosing unit had failed. The diesel exhaust system needs consistent DEF fluid flow, when the dosing unit malfunctions, the engine management prevents starts as a protective measure. The repair required replacing the complete dosing module since individual components are not serviceable. This is a significant repair as the unit is integrated into the exhaust system. The job involves draining the DEF fluid, replacing the dosing unit, and performing a system reset. Some warning signs before complete failure: Reduced DEF fluid consumption, Erratic level readings, Emissions control warnings, Delayed engine response The repair was substantial and required specialized diagnostic equipment. Would recommend having this checked by an authorized shop as improper repair can damage the SCR system further. Additional note: Simply adding DEF fluid will not resolve this if the dosing unit has failed. The system requires proper diagnosis and repair to function correctly.

Thanks for the detailed feedback on the DEF fluid issue. My SQ5 had nearly identical symptoms back in October. The emissions control warnings started showing up out of nowhere before the complete failure happened. After checking the DEF tank capacity and getting a full diagnostic, it turned out the dosing unit was indeed the culprit. The SCR catalyst system was intact but the faulty dosing module was preventing proper diesel exhaust treatment. Curious what the repair cost was in your case? Mine needed the complete unit replaced plus a system reset. Still monitoring if any other issues pop up with the emissions control components since the repair.

Thanks for the follow-up about your SQ5. The repair cost on my A6 was 250 Euro to fix the AdBlue output connection between the tank and dosing unit. The dashboard alert cleared immediately after the repair, and the emissions control system has worked flawlessly since then. Like your case, the SCR catalyst remained in good condition, the issue was isolated to the faulty connection. Having a proper diagnostic done was key, as it prevented unnecessary part replacements. The tank capacity readings are now stable and the system maintains correct DEF fluid flow. The repair has proven reliable with no recurring emissions control warnings or starting issues. Getting it fixed by a qualified shop was worth it, despite initial concerns about costs. The proper function of the entire SCR system depends on these components working correctly. Good call on monitoring the system. Regular checks of DEF fluid consumption and system performance help catch potential issues early.

Wanted to update on the final outcome of my SQ5 starting issue. After gathering advice here, I took it to a diesel specialist who diagnosed the DEF system properly. The dashboard alert showed the dosing unit failure, matching what others experienced. The repair involved a complete replacement of the DEF dosing module to restore proper emissions control function. Total cost came to 890 Euro including labor and parts. More than expected, but less than the dealer quote. The SCR catalyst system is now working correctly with stable DEF fluid readings. The fuel efficiency actually improved slightly after the fix. No more emissions control warnings or starting problems since the repair three weeks ago. Quick tip, the specialist showed me how to properly check DEF fluid levels to catch issues early. Much better experience than my previous repair attempts. The specialist explained everything clearly and fixed it right the first time.