Supra IV immobilizer fault

Whoa, immobilizer problems are the worst! Is the security light on the dash doing anything weird? Flashing rapidly or staying on solid? It sounds like the car isn't recognizing your key's signal. Did this start happening suddenly, or after any recent work on the car's electronics? I remember my old car wouldn't start when the battery in the key fob died. Have you tried the spare key? Worth a shot before you go down a rabbit hole!

The security light flashes rapidly. I haven't had any recent work done. I've tried the spare key, same result. Is it likely to be something besides the key itself, like the car immobilizer?

Rapid flashing definitely points to an immobilizer issue beyond just the key battery. Given you've tried the spare, it sounds like the car isn't 'talking' to either key properly. Could be a fault in the immobilizer module itself, or some kind of electrical interference. Honestly, messing with the immobilizer can be tricky and could brick your car completely. I'd strongly advise getting it checked out by a workshop with experience in car electronics. You don't want to risk needing a full immobilizer bypass, which is a whole different level of hassle.

Thanks for the advice. It was indeed a key programming fault. Workshop sorted it for 45€.