Motor 6 spins on power up

I have a problem when trying to arm on my hex. Motor 6 spins up aprox 5 seconds after power up and the APM 2.6 won’t arm. I thought it maybe a duff ESC but not matter which ESC is plugged into motor 6 socket the motor spins up. I have reloaded the firmware and reset the tx settings including failsafe and rebinded the rx but the problem persists. Is my APM knackered?