I've noticed that there is a round diagnostic port under the hood with a cover that screws on/off. Can this be used to test certain systems in my 280ce and if so does anyone know how. I've always wondered what that plug was for.

It's known as the x11 port.

I know pin 3 will allow you to use either a duty meter or oscilloscope (depending on fuel system) to measure the air / fuel ratio.

The fsm will detail more of its uses.

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-Typos courtesy of my mobile phone.