99 1/2 vw jetta wont disarm when unlocked

The alarm would cause the car not to crank, and the alarm horn would sound. You can try unlocking the car from the trunk lock to disarm the alarm. But crank and no start could be a bad ignition switch. Do the dashboard lights stay on or go out while the key is in the crank position? They should remain "on". Worn ignition switch could cause the car to crank but not start.

It sounds like the key sensor in the drivers door is malfunctioning, the drivers door is the only door that can enable/disable the alarm, if you try to unlock any of the others it will trigger the alarm.

Sometimes with my Kia I have to (without opening the door), cycle lock and unlock several times before the alarm is disarmed.

Have you tried this is succession, with all the doors locked:-

put the key in the drivers door, turn to lock and then to unlock, the alarm should disarm.

Also, if any of the door locks are jammed, the alarm will still activate when you try to unlock the drivers door.

If that fails and the alarm still activates, then it's a problem with the key sensor in the door.

Obviously you have a car alarm (do you?)
If until now was OK, then you have some problem with the alarm.
Try to disarm it and see what is happening.
Good Luck.

remove the key from the ignition and press the UNLOCK. its saying the alarm was triggered, eg if door was opened without first using the unlock on the remote. maybe when alarm was armed not all doors were fully closed. or you just reconnected the battery so the door was open without you ever pressing the unlock on the remote(which could be called DISARM the alarm)

Alarm box is behind the abs light panel and headlight switch, Its kind of hard to get to but there will be a 6 pin and a 10 pin, the other box is for the headlights, unplug the 10 pin harness and pit a jumper wire from the red wire to the red/black wire. Alarm is bypassed

ok so the same thing just happened with my jetta. So heres what worked for me: take off the door panel (drivers side) and you should see some melted or bunched wires together. just snip them and leave them be. It was either the wires or the sensor that messed everything up. This should work.

The immobilizer feature was tripped when the alarm went off. It sounds like the battery was indeed dead. My Nissan 300ZX used to trip the alarm when I jump-started it, although my Audi A4 does not. When the alarm is going off, you have to disarm it with the key or key fob first, or you won't be able to start it. Try putting the key in the door, lock it, unlock it, lock it again, and unlock it again, just to make sure you get a couple of arm/disarm cycles, then try starting it.

you can buy a factory vw key on ebay and have the dealer program it to your alarm. i had to do it that way when my key broke. it is cheaper than a new one from the dealer. type 01 jetta factory key on ebay.

Needs new drivers door lock actuator (P/N 3B1-837-015-AS)

you should be able to turn off alarm via key in drivers door if not test all doors to make sure they unlock. if any door will not lock that door latch will need replaced if work ok then try disarming alarm from trunk key or from the passanger door. if that works,you may need to have the key recoded to the car.