Macbook Pro fn key somehow locked ON

progress report ...

Well, actually, apart from ruling things out, there is no progress, in that I continue to have a computer that thinks the fn key is being pressed and the only way to get it not to think that is to actually press the fn key. Its as if the wires are crossed in the keyboard connection or else wherever in the underlying processing of basic inputs the system has been changed to believe that the fn key is always ON unless it is being pressed. This happens irrespective of whether I boot from the installed system (10.11.4) or an external drive, so it is inherent to the machine, not OS related.

So far, no one from the forum has offered any suggestions. Wondering if I am actually the first person ever to experience this phenomenon?

Just an update ...

I have plugged a borrowed USB keyboard from a Mac Pro into my MBP and it works correctly. Makes me think that there is an actual hardware issue with the MBP keyboard. Btw, the problem with the locked ON fn key also affects the operation of the trackpad on the MBP and this carries over to the functioning of a USB mouse connected to the Mac Pro keyboard.

I've just noticed that the Mac Pro keyboard doesn't have a fn key. Wonder if that could be relevant to the functioning of the USB keyboard.

more updating ...

I left the MBP on over night and it auto downloaded the latest el Capitan update, so it's now running 10.11.4, but the keyboard still works as if the fn key is ON.

I've also tried booting from an external drive into Mountain Lion, and the keyboard continues to behave as if the fn key is ON.

If it was a hardware problem, however, why would holding down the fn key make the keyboard work correctly? It's as if, on a laptop PC, the fn key is toggled ON, which can be done on a laptop PC. Is there a key combination to toggle the fn key on a MBP?

... well, I took my MBP into the Apple Store in town where a tech ran some diagnostic tests that came up with a failed keyboard as the cause. The tech agreed that it was strange that the fn key would behave as if it was being pressed when it is not, and as if it is not being pressed when it actually is, but apparently you can't argue with a diagnostic test. And, of course, a diagnostic test doesn't attempt to explain why the behaviour is happening, just that it is.

So for now it is a case of making do with a dicey keyboard.

If I get tired of the extra exercise for my brain and left little finger, Apple will change my keyboard for a new one for half the value of the MBP, or I could follow some on-line guides and attempt the replacement myself using a keyboard sourced from elsewhere. Any thoughts?

It's not as if I have spilt anything on it, so I remain bemused by the behaviour. ... sigh ...

... a final update (fingers crossed)

After leaving the MBP turned off and to its own devices for the past month, I booted it up on the weekend and discovered, much to my surprise, that the keyboard is once again functioning correctly. The reason is a mystery, but it has been consistent over the past two days of intermittently turning it on and then off after testing the keyboard. Who knows what goes on in their tiny electronic brains?