SMART check on SSD unreliable

I have a MacBook Air (11-inch, early 2014, now using Sierra), which came with an SSD hard drive, listed as "APPLE SSD TS0256F", manufactured by Toshiba. My Mac works fine, and I have no problems at all, specifically, nothing which would make me suspect that my SSD has problems. However, some utilities I have, like TechTool Pro, do a SMART check, which always tells me that my disk is failing. I am aware that some, perhaps all, SMART checking utilities may give false readings for some, perhaps all, SSD's - Toshiba's drives have been named as being generally untestable, unless perhaps Toshiba provided a firmware update (of which I'm unaware). I have tried to get help or advice from Toshiba, but no one there (US, Europe) is willing/able to help, sometimes saying that they would have to have the Toshiba part number, which I have been unable to determine (the Apple part number is apparently different) Can anyone tell me if there is a reliable SMART check for this drive, and if not, is there anything more I can do to see whether the SSD's OK or not. While I'd hate to replace a perfectly good drive in a perfectly good Mac, my data is too valuable to risk. (Actually, I have regular backup clones on an external drive, so I'm relatively safe if my boot drive failed, but it would be pretty bad if it failed when I were far from the backup, as is often the case.)