Ford Econoline E-250 Owner's Manual page 88

Locks and Security
If the key has been successfully programmed it will start the vehicle's
engine and the theft indicator light will illuminate for three seconds and
then go out. If the key was not successfully programmed, it will not start
your vehicle's engine and the theft indicator light will flash on and off
rapidly. If failure repeats, bring your vehicle to your authorized dealer to
have the new key(s) programmed.
To program additional new unprogrammed key(s), repeat this procedure
from Step 1 for each additional key.
88
2012 Econoline (eco)
Owners Guide, 1st Printing
USA (fus)