After using an improvised variant of this method to confirm that it was indeed an
electrical problem with no power reaching the spark plug wires, I had a closer look at my fuse
box, and specifically the «EFI» (fuel injection sytem) fuse.
GM is cooperating with the investigation and says the
problem is caused by a wire in the
electrical center similar to a fuse
box that can break because its location in a hot area near the engine makes it susceptible to stress.