VW diesel shutoff solenoid. This "On/Off switch" controls the flow of fuel. If it fails while you are driving, your car will either stay running when you turn off the key, or stop running and not restart.
The VW diesel shutoff solenoid is a quick 2-minute swap. If you keep a spare one in your glovebox you will be ready to keep on rolling if yours suddenly dies in a bad spot. Keep your Volkswagen Diesel running well with premium motor oil (required for TDIs) and fuel conditioner to keep moisture out of the fuel system.