I heard that while reprogramming an ECU, it is possible that 1 out of 10 times the ECU will fail and crash, then will be no good.

What is the cause of this? Is it the PC locking up while uploading data or the ECU just doesnt like it?

I have reprogrammed my ECU a few times but before I do anymore I would like some feedback about this.

 

It depends on a number of things. Noise in the electrical system, bad grounds, low car battery, low laptop battery, poor connection, other programs/devices interfering/interrupting the upload process on the laptop, human error, etc.

The reason the PCM does not work after a failed upload is because the program completely erases a chip at a time (out of 2 total) and then reprograms it. So if a reprogram halts during reprogramming, you've got a bad program and the pcm will not allow the car to start. In addition, you cannot just overwrite the corrupted program because the flash is corrupt and the chip cannot initialize itself to "talk" again.

These PCMs can be repaired though (I offer this service).

For what it's worth, I've reflashed mine on the road about 26 times a couple of weekends ago and had no problems, but even though I can fix them, I'm still nervous.