Renaul Megane II does not turn on. If it starts

Besides what you've already checked, have you considered the vehicle's immobilizer? How often does the LED located in the center of the instrument panel flash, or does it turn off when you press start?

Hey, I think I know what the problem is. I had a very similar issue once. It started with ignition problems; it would crank but take a while, or the idle would be unstable, or it would start and the starter motor would keep working, and then it wouldn't start at all. A strange code came up on the scanner. It took me a long time to find the problem, which seems to be the same as yours. The ignition relies on a signal sent by a small device—I don't know its technical name, but it's like a little black box at the intake manifold. It controls the idle and sends the different throttle positions, something like the TPS. You can easily identify it. I took it apart and opened it; it was full of carbon buildup and dirt. I cleaned it, put it back together, and the problem was solved. Try it, and please let me know how it goes. I almost went crazy looking for that problem. Good luck with it!.

The device or black box is called a TAC or ECT, and you must be very careful when handling it. It's best to handle it with the battery disconnected, as its communication is via a multiplexed network.
Have you checked the CKP signal?

I already sent the fuse box to be repaired. As soon as it's ready and I install it, I'll let you know what happens. Thanks.

Okay friends, I fixed the fuse box, the car starts but it still has an idle problem; when I press the accelerator it's like it's choking. I'm going to check the TAC or ETC as CRAZY and DANINOVATO recommended. I'll let you know how it goes.