Macbook air will not start - Circle/slash symbol

My experience as a tech and consumer is that when a RAM test indicates the RAM passed the test it means it passed this time but the RAM might still be iffy. But, when the RAM test indicates a failure it is almost always correct. I think you are going to wind up making a call or trip to an Apple service center.

But there is one more thing you might try. You indicated that you attempted to reinstall the OS but you didn’t indicate that you had first formatted the drive. If you attempted to install the new OS overtop the original OS maybe a damaged directory explains the failure. And if you did format the drive first it may be that you need to do a network recovery and first partition the drive. It’s worth a shot doing a network recovery and repartitioning the drive.. Otherwise, as the Apple Rep said, you did a very complete job of handling the problem yourself.

Partitioning isn’t the same as format and erase. Once you have booted into Internet Recovery choose the Disk Utility app. Click on the drive icon probably labeled Apple SSD - not the one under it probably labeled Macintosh HD. Then select the Partition option. Choose one partition - that will replace the current partition and then OK. Assuming that works, then back out and download & install the OS.

If that fails we’ll know the RAM really is defective. If it succeeds then go ahead with the migration from your backup. My entire idea is based on the (remote) possibility that your problem stems from a damaged directory or other disk error and partitioning is the best way to make sure that all gets set correctly. Simply migrating isn’t likely to do any good. Again, I think the chances are your RAM is bad but I’ve had too many instances where a colleague or customer came back with a working computer when I pronounced it RAM dead and the common thread was their claim of the problem being the hard drive.