2002 Ford Explorer lock cylinder replacement, antitheft sys

Do you mean it isn't programmed? Matched to the car? Or it isn't a key that can be matched?

KEY PROGRAMMING USING DIAGNOSTIC EQUIPMENT
Special Tools
Fig. 22: Identifying Key Programming Special Tool
Courtesy of FORD MOTOR CO.
Key Programming
NOTE:This procedure is used when a customer needs keys programmed into the system and does not have two programmed ignition keys available. This procedure is also useful when programmed ignition key(s) have been lost or a new ignition switch assembly as been installed, and it is desired to erase key(s) from the passive anti-theft system (PATS) memory.

NOTE:This procedure will erase all programmed ignition keys from the vehicle memory and the vehicle will not start until two keys have been reprogrammed to the vehicle.

NOTE:Two PATS encoded keys with the correct mechanical cut must be available to carry out this procedure. One or both of them may be the customer's original keys.

NOTE:If additional keys are to be programmed, refer to KEY PROGRAMMING USING TWO PROGRAMMED KEYS. If the remaining keys are with the customer and are not available with the vehicle, then instruct the customer to refer to the Owner's Guide under the "Programming Spare (SecuriLock(R) North America, Safeguard(R) U.K. Or PATS for all other markets) Keys Procedure" for instructions on programming the remaining keys.

Turn the ignition switch from OFF to RUN.
Using the diagnostic tool, follow the SECURITY ACCESS to obtain security access. For additional information, refer to ANTI-THEFT SECURITY ACCESS.
From the diagnostic tool menu, select IGNITION KEY CODE ERASE.
NOTE:Do NOT select any additional commands from this menu.

Turn the ignition switch to OFF and disconnect the diagnostic tool.
Insert the first encoded key into the ignition lock cylinder and turn the ignition switch to RUN for three seconds.
Insert the second encoded key into the ignition lock cylinder and turn the switch to the RUN position for three seconds.
Remove the second encoded key from the ignition lock cylinder.
The vehicle should now start with both ignition keys.