Back in 2018 my GDI would not start - I found the cylinder head of GDI has a secondary earthing wire fixed to the rear of the head low down just above the join with the main block on the left hand side (passenger side in UK). Photo from the rear left hand side of engine shows the wire (pinky red, running vertically upward) bolted via a ring connector to the head. Not a good photo but best I could get as access is poor and flash bounces off all sorts.

When my GDI would not start (engine turned, petrol pump worked etc) this bolt was found slightly loose – when tightened to ensure good earth for cylinder head all was well.

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Richard
+4 GDI Connaught Green, 2014
Magnolia