I think this will be refering to the start of injection coil located on one of the fuel injectors, injector 4 I believe. Unplug it from the loom & check its resistance with a meter, should show about 1000 ohms at a guess. It detects when the injector opens & is used by the ECU to make a fine adjustment to the injection timing.

Wouldn't have thought it would have stopped it from running though.

Check the glow plugs & that you have fuel getting to the pump ok.