If I'm not mistaken (and I may be) OS X will not allow you to secure erase an SSD. Hard drives and SSDs retain data differently, and your Fusion drive has one of each.
With a hard drive, a reformat without secure erase essentially only clears the index files so when the OS sees the drive, the index looks empty and it can freely write data anywhere on a drive. On a hard drive, under normal operation, the OS never really normally erases or clears a sector (unless secure erase is done) it just marks one that had data but has since been erased as being available for writing, and the data in the block is still there. When it comes time to write to that block, the HDD just overwrites the used block with new data. That's how file recovery software works, it rebuilds an index from the data leftover on the drive. With secure erase on an HDD, instead of just clearing the index so the drive looks empty, it clears the index and then overwrites all the blocks on the drive, either with random bytes of junk data or blocks filled with binary zeros. I think Disk Utility uses the binary zeros approach for a single pass secure erase, but don't quote me on that. It may switch to a binary pattern during a multiple secure erase pass.
An SSD is different. When an SSD erases a block it doesn't just mark it as ready for use leaving the data in place like the HDD did, it must first completely erase the data block before it can become available for use again. Since NAND flash blocks in an SSD have a limited number of write/erase cycles before they can't be used anymore, every time one of these blocks is erased and written to, the number of writes available for each block goes down. I suspect this process (secure erase) isn't allowed because each time such an operation is performed it's decrementing the write/erase counts for every NAND block on the SSD and is basically, potentially, shortening the life of the SSD.
The following tactic may be of use to you (then again, it might not):
http://osxdaily.com/2013/04/22/secure-erase-mac-ssd-drive-recovery-mode/
That trick might work with the Fusion drive, but if you notice the date on the article it's a few years old so who knows.
Another thing you may need to do is first break up the Fusion drive into two non-Fusion components, which needs to be done using the command line version of Disk Utility and then do the secure erase on the drives.
If you really want to make sure that the Fusion drive itself is bad, you could get Scannerz because it will segregate the SSD and HDD components in the Fusion drive separately and test them by component, letting you know which one is bad. To the best of my knowledge, it's the only tool on the market that can do that. More info on that can be obtained at:
http://scsc-online.com/Scannerz.html .
With all that said, because you have Apple Care, you may want to contact them and see if they'll return any bad components for you without necessarily doing all the work I've just outlined. First, you've already paid for it, and second, I see no reason why they would want keep a bad HDD or SSD.
