Hi everyone, I'm new to this forum in need of some guidance please. I have a 2007 Q7 that has been running without an issue. Recently the kids managed to drain the battery completely so it wouldn't even crank over. Had the car relayed and they tried a booster pack to no avail but easy start did work for a short while, but when doesn't that stuff work! 

I have tested every fuse and relay in the car, changed the battery with a brand new one including replacing a main fuse on the battery that can blow apparently. Anyway the car will crank over but refuses to fire up. Interestingly I'm not hearing a humming from the fuel pump but have taken the pump out of the tank to test which it spins given a 12v feed. I'm also getting 12v at the fuel pump connector when cranking. 

What I dont understand is how draining the battery can stop it from turning over when on boost, fully charged or even a brand new battery! 

Any ideas would be much appreciated. 

Injector out Fuel coming from injector? (put in a pop bottle when testing)

Check earthing - earth straps from block to body - common issue on Q7 

Immobilizer circuitry? Appreciate it should not crank but have you experienced a partial fail?

Check all critical loom connections - jump starting is harsh on ECU etc - any faults logged in the system?

I’ve heard of a fault on Audi diesels which can happen when you try to jump start them on a dead battery. There’s a relay that fails the tell tale sign is the glow plug light doesn’t come on when you put the ignition on. My mate told me about it ages ago but can’t remember where he said it was 🤔 I think he said it was somewhere by the steering column but I’m not sure. See if your light is coming on or not and if needed I’ll message him to find where it is.