When I bought my used 2003 Taurus I got only one key.
I went to the locksmith to get two more. He said there is a chip inside the key and he has to program it. One key would be 60$ and two would be 85.
OK, I got two for 85$.
I tried all three keys and they worked.
A month later I couldnt start the car with one of the copied keys. The theft light would start to blink.
I disconnected the battery and connected it again and the car would start.
The following days this happened a couple of times. So I thought maybe its the copied key.
I started to use only the original key. And I had no problems for more than 6 months.
Three days ago I used one of the copied keys. everything was OK.
Yesterday my wife drove the Taurus to a store using the original key. After shopping she tried to start the car, but couldnt. I got there, disconnected the battery and connected it again and the car would start.
I drove it, parked and started a couple of times. everything was OK.
But later that day I couldnt start it again.
So looks like the copied keys mess up the whole car.
I dont know if its a coincidence, but the first time the problem occured was a day after I disconnected the battery (to turn the check engine light off).
I havent asked the locksmith yet, if he knows what the problem could be.
Does anyone had the same problem?