Lost car key and fob,I have the valet key but starts for a few second then shuts down and alarm goes off How can I disarm the alarm?

You could try disconnecting battery, leaving overnight and then inserting ignition key and switching on before reconnecting the battery.

It has sometimes worked for me but in reality it seems you probably have an alarm immobiliser fault you need specialist help with.

Yes you need to have a dealer test the system for fault codes.

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.

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

most aftermarket alarms have a valet button and in the case of lost or broken key fob the button can be held or pressed when the ignition is turned on the turn alarm off. if it is a factory installed system usually opening driver door with the key will disarm it

The 2000 Dodge Intrepid is equipped with an alarm system that is triggered when unauthorized entry is attempted. In some situations, such as having the Intrepid valet parked or when it is being serviced by a mechanic, the alarm system needs to be disarmed so it doesn't go off. Disarming the alarm will prevent it from being activated until it is armed again.
Difficulty: Moderate Instructions

1. 1 Insert the key into the ignition.
   
2. 2 Turn the ignition switch to "ON." Do not start the engine.
   
3. 3 Press and hold the valet/override button within five seconds of turning the ignition on. The valet button is located beneath the steering column.
   
4. 4 Release the valet/override button after the security indicator light turns off.
   
5. 5 Turn the ignition switch off.
   

have to do it in this order

You will need to find a push button or valet switch (hidden somewhere under the dash), This will help you in turning off the alarm. Once you find this button, turn the ignition key to the run position, do not try to start the vehicle, and push down button for about 3 to 5 seconds this will shut alarm off. Good Luck.

was it a professional or shade tree install? the following is off vipers website:
disarming without a remote control
This feature allows you to disarm the security system without the
Remote Control should it be lost, damaged, or disabled. In order to
disarm the system without a remote remote control, you must have
the vehicle’s ignition key and know where the Valet button is
located. Be sure to check with the installer for the location and the
number of presses of the Valet button required to disarm the system.
To disarm the security system,
turn the ignition to the ON position.
Press the Valet button the
preset number of times (one to
five times) within 15 seconds. After five seconds the system will
disarm. If the system does not disarm, you may have waited too long;
turn the ignition off and on and try again.
if that doesn't work try this other

valet mode
You can prevent your security system from automatically arming
and triggering by using Valet Mode. This is very useful when
washing the vehicle or having it serviced. In Valet Mode, the
security system will not arm, even with the Remote Control, but
all convenience functions (door locks, trunk release, etc.) will
continue to work normally.
To enter or exit Valet Mode:
1. Turn the ignition on.
2. Turn the ignition off.
3. Press and release the Valet
button within 10 seconds.
The status LED will light solidly if you are entering Valet Mode,
and it will go out if you are exiting Valet Mode.

it sounds like you may have an aftermarket alarm system. it can be removed completely from the vehicle. if it is a factory system the key fob or unlocking the door wit the key should disarm the system. if they dont you may want to have a dealer or reputable shop check the system