C4 AdBlue System Failure

Sounds like a classic AdBlue issue. Is the dashboard alert constant? And have you checked the DEF fluid lines for any visible crystallization? The system is quite sensitive to pressure and flow.

Yes, the dashboard alert is constant. I haven't checked the DEF fluid lines closely. Could a defective AdBlue output from the tank to the dosing unit prevent the engine from starting, like I've read?

Yes, a defective AdBlue output is a common culprit. A blockage or faulty pump often triggers the 'no start' condition due to emissions control regulations. Given the symptoms, I'd suggest a workshop visit to confirm the diagnosis and avoid further complications. Expect a cost of around 250€ for the repair.

Thanks for the help. I'll book it into a workshop to get the AdBlue system checked. Good to know my suspicions about the AdBlue output and the repair cost were on the right track.