hi, i was doing a downgrade from 3.41 to 3.15 but during the process it was taking forever and stupid i turned the ps3 off, now every time i turn on the ps3 after 10 seconds it turns off.
Had the same problem ages ago. Put modified 3.41 .PUP on USB flash drive in PS3/UPDATE and plug it into the rightest USB port on your PS3. Just start PS3 and wait. If this doesn't work, use the original .PUP from 3.41 and give it a try. One of these options worked for me.