Peugeot 607 Engine Control Malfunction

Check your battery cable connections. A bad connection can cause this.

Thanks! What exactly is a 'bad connection'? What should I look for when checking the battery cables?

Corrosion, looseness, or damage to the cable itself. If those look fine, go to a workshop.

Thanks for the suggestion! You were right, the battery cable connections were the problem. The local garage sorted it. Final repair cost was 155€. Thanks again!