The operation of the key in relation to the vehicle varies depending on the make and model of the car, but if the batteries are brand new it may be that the issue is with the key detection system within the car. Some vehicles will enable a safe mode if the battery is low, so you could try holding the fob against the steering column when starting the car, or there may be a specific method outlined in your vehicle's handbook. Failing this, we would suggest contacting an automotive locksmith or independent specialist for your make of car that may be able to help.-