Strange immobiliser problem, Rover 45

Not mine but a friends 45. Basically he left his car keys at work so came to use the spare set. The car unlocked with the fob and de-activated the immobiliser. The car started fine but after a couple of seconds the immobiliser light started flashing a bleeping then the engine cut out. Pressed button on the fob and the engine re-started instantly but the same thing kept happening over and over again.



After panicking the car was dead he got the original keys back from work and now the car runs and drives fine. Why is the spare set of keys causing this problem?

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