Ford Expedition 2006 Owner's Manual page 546

Locks and Security
6. To program additional new unprogrammed key(s), repeat this
procedure from Step 1.
If successful, the new coded key(s) will start the vehicle's engine and the
theft indicator will illuminate for three seconds and then go out.
If not successful, the new coded key(s) will not start the vehicle's engine
and the theft indicator will flash on and off and you may repeat Steps 1
through 5. If failure repeats, bring your vehicle to your authorized dealer
to have the new spare key(s) programmed.
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2006 Expedition (exd)
Owners Guide (post-2002-fmt)
USA (fus)