2003 Ranger theft light

I recently had a copy of my ignition key made to have an extra.
It worked okay in the door but would not work in the ignition.
When I removed it from the ignition slot the THEFT light came
on and is now always blinking on when the car is parked. It does
go off when running. How do I reset the --whatever-- to get the
theft light off when parked? I tried disconnecting the battery for a couple of hours but it did not help. I don't want to take it to the dealer.
Any ideas? Thanks for help.

the theft light is supposed to blink always when the key is off. you will not be able to start your truck with that key until you program it. it has a rf chip in the key and the reciver is in the steering colomun. instructions are in owners manual how to program a 3rd key. if you only had one before you might need to call a shop that installs alarms to program it for you. you need 2 programed keys to do it yourself.