Peugeot 408 Cold Start Issue

I had a similar issue with my 408: hard starting and rough running when cold. Have you tried fuel injector cleaner or checking the fuel rail pressure? Maybe consider an injector test to rule out a defective injection valve. Any difference based on the ambient temperature?

I haven't tried fuel injector cleaner or pressure tests. What does the injector test involve? Can it be done independently, or is specialized equipment required? I will pay more attention to ambient temperature.

The injector test checks the spray pattern and flow rate of each injector, usually requiring specialized equipment best used at a workshop. Given your symptoms and mileage, a defective injector is plausible. I took mine to the local garage, who confirmed this and quoted 515€ for the repair. To save time and money, go straight to them.

Based on your recommendation and the symptoms, I've scheduled an appointment with my local mechanic. I will share the diagnosis and resolution after inspection, including your suspected root cause. Thanks for your support!