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All Honda S-MX have central locking, but not all are fitted with remote central locking.
To tell if yours has remote central locking (and the key fob is probably lost) simply look where the central rear view mirror is, on the cover that attaches it to the roof there will be a small 15mm black dome. This is the receiver.
Not all key fobs will work with the S-MX, and most that are available from Japan are for the facelift model that uses a different keyless system.
The only key fob that works with the S-MX (96-98) looks like the one below, it has a flat grey button.
1. ENTER the vehicle and CLOSE all doors. (Each of the following steps must be completed within five seconds of one another. If you take longer than five seconds between steps or make a mistake, you must start the entire procedure over.)
2. INSERT the key into the Ignition, HOLD one remote in your left hand with additional remotes close by.
3. PERFORM each of the next three (3) steps in order, then repeat all three (3) steps two (2) more times for a total of three (3) times each.
4. TURN the Ignition to the ON position, marked “II” on the Ignition.
5. PRESS the LOCK button on the remote for approximately 1 second. (Say “Mississippi”)
6. TURN the Ignition to the OFF position, marked “I” on the Ignition. (REPEAT Steps 4-6 three (3) times)
7. TURN the Ignition switch to the ON position, marked “II” on the Ignition.
8. PRESS the LOCK button on the same remote. The door locks should cycle indicating the vehicle has ENTERED Programming Mode.
9. PRESS the LOCK button again on the same remote. The door locks should cycle again indicating the vehicle has successfully PROGRAMMED that remote.
10. For programming additional remotes to your vehicle, REPEAT Step 9 with each additional remote one after another. Step 9 must be completed within ten (10) seconds for each additional remote. You may program up to three (3) remotes to your vehicle.
11. TURN the ignition to OFF.
12. Test all remotes. Programming is now complete.