Avensis Diesel Engine Fails to Start

That sounds frustrating. The fact that the OBD-II reader can't connect to the ECU is a key clue. Before diving too deep, have you checked the cable connections of the battery itself? Sometimes a loose or corroded connection can prevent the car from starting, even if the battery seems fine and other electrical systems are working. It's a simple thing to check and rule out before looking at more complex issues like the ECU or fuel injection.

Thanks for the suggestion about the battery cable connections. I visually inspected them and they seemed secure. What specifically should I be looking for beyond looseness or corrosion? Are there tests to check the connections?

I mean that the cables are tightly connected to the battery poles. Also check if the cables are not damaged. If the connections are clean and tight, and you're still having problems, it might be best to take it to a workshop, as it could be an ECU issue or something related to the fuel injection system.

I appreciate the advice. It turned out the cable connections of the battery were indeed the issue. After cleaning and properly tightening them, the engine started without any problems. The workshop charged me 155€ for their service. Thanks again for pointing me in the right direction.