FORD 2003 F-150 v.4 - Manual (Page 90)

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, or stay
on for more than three seconds. If failure repeats, bring your vehicle to
your 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.
Locks and Security
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