FORD 2001 Explorer - Manual (Page 120)

5. Within 20 seconds of removing the second coded key, insert the new
unprogrammed key (new key/valet key) into the ignition and turn the
ignition from 3 (OFF) to 4 (ON) (maintain ignition in 4 (ON) for at least
one second). This step will program your new key to a coded key.
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. If failure repeats, bring your
vehicle to your dealership to have the new spare key(s) programmed.
Controls and features
120