My 1999 406 estate won,t lock. when you push the remote key it clicks to lock but clicks again to unlock on its own. please help

Climb into the vehicle and close the door. Insert the key into the ignition but do not turn on the vehicle.
Hold down the "Unlock" button on the driver's door while turning the key to the right to the "Accessories" position. Continue holding down the "Unlock" button.
Turn the ignition key back to the "Off" position while continuing to hold the "Unlock" button then return the key to the "On" position again. Then return the ignition key to the "Off" position. As you complete this sequence, the doors will lock, and the horn will sound, indicating that you've entered the programming mode. At this point, release the "Unlock" button on the driver's door.
Push both the "Lock" and "Unlock" buttons on the key-less remote and hold them for roughly 15 seconds. Release the buttons when the locks click again.
Insert the key into the ignition and turn it to the "On" position. The horn will honk once more. This indicates the programming has been successful.
Remove the key from the ignition. Climb out of the vehicle and try locking and unlocking the doors with your remote key.
Tip

• The H3 uses a Tech ll code system that often cannot be re-programmed without the help of your local dealer. If you are unable to reset your key-less fob using the steps above, your local GM dealer will be able to help.

Check fuses, remote control battery, try to push "unlock" button 5 or more times. If it won´t help, you need Tech-2 diagnostics.

The fob may need to be reprommed to the car. Do the following:

Grab your working remote. Push the LOCK button five times, allowing exactly two seconds to pass between each press of the LOCK button. Insert the ignition key and turn it to RUN. Push and hold UNLOCK on the working remote for 6 seconds. Push and hold PANIC for one second while continuing to hold UNLOCK. Release both buttons at the same time.
Push any button on the remote to program it. Then push any button on the remote that was already programmed. It has to be reprogrammed. The remote is now programmed to the vehicle. Turn the key to OFF.

lost the other fob>>> ?????
never own a car with 1 fob.
and if one fob messes up, the battery inside is dead. (or worse)
if just lock fails the button is bad, if one fob false, and they do
its just cheezy switch. (can be cleaned)
use the other FOB, if it fails, post.

this is a problem that can appear after your car battery died, cause the car didn't reset propely, so taking the battery out and putting it back in or letting your car die again would do the trick. I tried a lot of reprogramming procedured for my remote key, none of them worked, what did the trick though was simply unlocking the doors via the all door unlock button on the passenger side door, so simple.

If you don't have a key, replace the lock with one that has a key with it. If you can get it to work with the remote, why not leave it alone. When you replace it with a lockset with a key, you may not get a fob that works with it. It's a catch 22 thing in progress here. If I do this, I can't have that. If I do that, I can't have this.

Try unlocking the car with the key on the passenger side and locking it again, that resets the alarm. Use the key to unlock the car, same problem on my Audi estate of the same age. Found out later one of the windows was still open and the sensor detected that.
Manu

YOUR PROBLEM IN THE FRONT PASSENGER DOOR CENTRAL LOCKING UNIT.OR YOU HAVE FAULTY DOOR LOCK ELECTRIC ACTUATOR OR SOLENOID.

I have an A4 and did this to re-program my key:

1. used the key to start the car...then turned it off
2. put the key in the driver door (door closed)
3. pressed the lock button on the key fob once
4. turned the key to lock the door.

This is basically the steps in the owners manual, but the car must be started with the key first, and the key must be in the door lock before pressing the lock button.

you either have a faulty central locking motor or the wiring harness may be damaged.
Remove the door panel and check if you have a live feed to the central locking motor when you use the fob to lock or unlock. If you have power, then it will most likely be the motor that is at fault.