My (FAT) PS3 refuses to downgrade.

Every time I try and downgrade it fails with a constant green blinking light (the console does not switch itself off) with the following error seen in the log.

"failure manufacturing updating FAILURE(0x8002f14e)"

I can get the console back by flashing back to 3.50.

I have tried the following firmwares

3.41
3.42 (modified)
3.21
3.15

Any ideas?