The alarm system has activated and we cannot disarm the system via the Remote Key Fob or by key at the doors or hatchback. Tried disconnecting battery same problem. When the doors open the system activates, car no run feature is enabled.

Question edited for clarity. The word 'faub' does not exist - 'fob'.

Instructions for resetting the anti-theft system. The negative cable is always the one to disconnect first and reconnect last. You could also just phone for a mobile vehicle alarm installer / locksmith, or a mobile auto electrician.

In the link it say 5 times and then start the vehicle.

https://www.google.com/search?q=2000+mitsubishi+montero+sport+reset+security+system

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Fob didn't work so the alarm immobiliser was armed and active when the locksmith opened the car - with predictable results.

Fob still not working - broken or needs new battery. Have you fitted a new battery into the fob?

The alarm immobiliser will probably remain armed until the doors have been locked and then unlocked with the fob.

There is a small chance the system can be fooled into thinking it has been disarmed by disconnecting the battery (overnight if poss) and then with the door open and the ignition key in and turned on, when the battery is reconnected it might be possible to start the engine...

Door Key Switch
The BCM monitors the door key switches in order to disarm the CTD system. When the lock cylinder turns in order to unlock the door, the switch grounds the disarm input to the BCM.
The key lock input to the BCM is probably missing , BCM doesn't see it . So it think's your breaking into the vehicle. Did you try the passenger door to see if it will disable the alarm .
The BCM - body control module ! Electronic -computer !
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Content Theft Deterrent (CTD) Does Not Disarm with Key Lock

The factory service repair manual has a 23 step diagnostic procedure . An you would need a factory scan tool to find the correct fix.
Turn ON the ignition with the engine OFF.
With a scan tool, observe the door key switch parameter in the Body Control Module (BCM) security data list.
Does the scan tool display INACTIVE?

Locking the Vehicle Without Arming the System
Use one of the following procedures in order to lock the vehicle without arming the system:
• Procedure A -- Lock the vehicle using the manual door lock knobs. Close all the doors.
• Procedure B -- Close all the doors. Use the key or the power door lock switch in order to lock the door. The doors must be closed prior to locking the doors for this procedure.
Disarming the System
In the arm initiated mode, the following conditions are true:
• The security telltale is ON.
• The doors are open.
In order to disarm a system in the arm initiated mode, perform one of the following procedures:
• Press the power door lock switch to the UNLOCK position.
• Press the door unlock button on the key fob.
In order to disarm a system in the arm delay or the armed mode, perform one of the following procedures:
• Use the key in order to unlock one of the doors.
• Press the door unlock button on the key fob.
Deactivating the Alarm
In the alarm mode, the following conditions are true:
• The horns sound intermittently.
• The headlights and the park lamps are flashing.
In order to deactivate a system in the alarm mode, perform one of the following procedures:
• Use the key in order to unlock one of the doors.
• Press the door unlock button on the key fob.
• If the vehicle control module (VCM)/powertrain control module (PCM) receives a valid Passlock™ ignition key rotation.

Disarming an Armed System/Silencing an Alarm
If system arming has been requested by the keyless entry transmitter, it must be disarmed.
Important: Using the power door unlock switch or manually unlocking the doors does not disable the CTD armed mode. Also, disconnecting the battery or pulling fuses does not disable the armed mode, since the BCM stores the CTD mode status in memory.
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To disarm the CTD system in arm pending mode, SECURITY indicator flashing and doors open, perform 1 of the following:

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Press the UNLOCK button on the keyless entry transmitter.

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Insert a valid key into the ignition and start the vehicle.

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To disarm the CTD system in the armed mode:

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Press the UNLOCK button on the keyless entry transmitter.

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Insert a valid key into the ignition and start the vehicle.

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To disarm the CTD system when activated, alarming mode.

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Press the UNLOCK button on the keyless entry transmitter.

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Insert a valid key into the ignition and start the vehicle.

The alarm operation is a function of the body control module. I'm not aware of any owner procedure to disable it. You could disconnect the horns, but that creates other problems. If you lock the car manually with the locks on the window ledge, NOT the electric switch, NOT the remote and NOT with the key, the alarm system is not supposed to activate and should not trigger. The red light on the dash will not be flashing if the alarm is not set. It isn't a perfect solution but it may be a workaround.
Unlocking the doors with the key is supposed to shut off the alarm. Have you tried that?
Do your remotes work consistently for other functions? Maybe the batteries are getting tired.
24 7 Locksmith Williamsville Ny 716 200 1030 Locksmith Williamsville Ny

stick the key in the door (driver front) and hold to unlock

You should be able to disable the alarm by putting the key in the door and turning it, or by putting it in the igintion switch and turning it. One of the two should disarm the alarm. If that will not work, then you could disconnect the battery, call the dealer and get a key fob for the alarm system.

Find the alarm and disconnect it, or locate the fuse to the alarm and remove it. Then you can charge the battery without all the noise. Once the battery is charged, hook the alarm up and disarm it with the remote.

The Simple Act of Locking the Driver side Door WITH KEY Arm the Factory Alarm. And UNLOCKING Same Door with KEY Disarms it The FOB < Rmote for Alarm Just Honks Horn and Flashes Lights, But it Does the SAME ARMIND & DISARMING. I Hope this Helps You out !!

Sounds like you have an aftermarket alarm with starter disable. By disconnecting the battery the alarm reset and the defualt setting is usually to set the alarm automatically (active arming). You have three options..
A. If you have the key fob that went with the alarm you can disarm the alarm.
B. If no key fob is available you can disconnect the battery again, turn the ignition key to the on position and reconnect the battery. This usually doesn't allow the alarm to set and allows the car to start.
C. Remove the aftermarket alarm system. This is usually the long term fix i recommend if the fob isn't available. Option B should get you going though.