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BRZ not starting - suspect immobiliser

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So my BRZ has seen very little use over the past weeks and whilst the battery is relatively new even it won't hold a charge forever. I've had to jump start it a week ago and thought it would be okay for another week, however when I went to start it there was no response to the key fob at all. I opened the door with the key and the alarm went off (though not especially loud) it still wouldn't recognise the key and any key button presses.

Connected up jump leads and had a good connection, I could even operate the electric windows! But no cranking.

Battery is new in the key fob, it's as if the alarm/immo isn't recognising it. Car is at work so only tried with one key fob, the one I had, so I could try the other, but it's a pain getting down there and would need to take jump leads etc. What I mean is when I go I'd like it to be to get the car started so if there is a known procedure to get the alarm/immo working and/or the key recognition that would be a massive help.

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Try to lock in and unlock it with the jumper pack connected so the alarm does an arm disarm cycle.

Also try and keep the fob as close to the start button as possible. 

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I had something similar recently. My battery had died, so I disconnected it and bought a new one.

When I fitted the new battery, I couldn't get the alarm to disarm or the siren to stop sounding. The keyfob just wouldn't disarm the alarm. What seemed to finally cure it was pressing the start/stop button three times (so basically fully on and then off) - at that point I was able to use the keyfob to disarm the alarm and I could then start the car.

The other thing which may work (although it didn't for me...) is to use the keypad at the rear of the glovebox to enter your alarm code.

The "quiet" siren that you heard was probably because it was being powered by the backup battery within the siren - this is a Thatcham requirement.

Holding the fob next to the start/stop button is only useful if the battery in the fob is dead - it then uses NFC to let the car know that the keyfob is present.

I have a GT86, so the BRZ may be different is some/many/all ways!

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4 hours ago, McDude said:

That wouldn’t explain why it won’t crank with jumper leads. 

Yes, it does.  A battery charger typically pushes out 8-10A which is not enough to crank the engine.  Cranking current from the battery can be 100A or more for a couple of seconds.

Further, a bad battery is going to absorb a lot of the juice from the charger to little effect except producing some heat.

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