Equus No Start

Have you checked the battery cable connections? A poor connection there can cause all sorts of electrical gremlins, including preventing the engine from starting. Even though the diagnostic tool reports other things, these tools only find a correlation, not necessarily the root cause. Make sure they are clean and tight. By tight I mean, try moving the cable by hand. If you can wiggle it, it's not tight enough.

Good call on the battery cables. I'll double-check them. But even if they're a bit loose, would that completely prevent the car from starting? I mean, wouldn't it at least try to crank or something? Like, is there enough juice to run the starter? I am not sure what an ECU is doing at start.

A bad enough connection can indeed prevent any cranking. The ECU (Engine Control Unit, which is the computer that controls your engine) needs stable power to function correctly, and a poor connection can cause voltage drops and signal interference. It's the easiest thing to check before assuming the worst. If the connections are solid and clean, you're probably looking at a more complex electrical issue. At this point it is likely you need to take it to a shop.

You nailed it! Turns out, the negative battery cable was corroded something fierce. Cleaned it up, tightened everything down, and she fired right up. Cost me 155€ for the cleaning. Thanks a million for the advice! I was all ready to start tearing into the fuel injection system!