Key programming

Writing your key's transponder identity into the car's immobilizer so the two recognize each other, and where there is a remote, registering the buttons with the body control module.

They are two separate operations. A remote that locks the doors but will not start the engine is a key that was half-programmed.

Many vehicles can be programmed through the car's own diagnostic port. Some manufacturers require an online security authorisation, and some restrict it to their dealers entirely.

This varies by make, model and year, so it is a question to ask about your specific vehicle rather than one with a general answer.

A number of vehicles will only accept an additional key if an existing working key is there to authorize it. On those, going from one key to two is routine and going from zero to one is a much bigger job.

Which is the argument for making the spare before you need it.

Where no key remains, programming normally erases the previously stored keys so anything still in circulation stops working. That is the correct outcome if a key was stolen.