Reset alarm system

After Market alarms are always full of issues especially when the batteries are disconnected of the car batteries are disconnected.

Find a Viper Alarms installer and take it in for reprogramming with their pro grade scan tool computer.

if door is not locked when u reach in wont activate alarm --try lock dr and use key in lock and open dr alarm should activate in a few seconds and will shut off when key put in ignition or key fob shut off -if it does not set it off --check ur settings thru ur dash information if setting is as you want than may have to have system reflashed at dealer

Ford sold a Ford branded alarm through the parts department. These sometimes were a dealer installed option on new cars. Mine would randomly stop responding to the remote and I would have the same symptoms as you describe. I had it removed and the original equipment factory system still worked. Be warned, if you have the Ford "accessory" alarm, your key fobs (remotes) are different and the method to program them is different from what you will read online. You can tell if you have the system by looking at your key fob. The standard Mustang key fob is a 4 button (lock, unlock, panic, and trunk). The accessory alarm uses a 3 button remote and clicking unlock twice would pop the trunk. I really wish I had a picture of the remote because there are 3 button Ford key fobs for cars without a trunk release although I don't think you should have one on a Mustang. Long story short, if you have the accessory alarm, you can solve your problem by having it removed but you will need new 4 button key fobs to remotely lock/unlock your doors

Sounds like the security system is armed. It is designed to disable the starter. If you don't have a key fob for the alarm, try locking and un-locking the driver's door with the key to disarm. If that doesn't work, look for a very small black button or toggle switch that will either be taped to wiring under the driver's side or the dash, or mounted to the very bottom of the dash above the driver's legs.

It's normal for the alarm to go off if it was set with the key fob and then unlocked with the metal key. The manual should explain this.

I take it the key fob doesn't work Try disconnecting the battery for 30 minutes and see if that will reset the alarm. You may need the key fob to reset.

this is how most nissans program the key fobs.
get in the car and manually lock the drivers door with the door switch.
put the key in and out of the ignition about 10 or 12 times until the hazard lights flash once.
turn the key to the ignition setting so all the dash lights are on,but do not start the car.
click the lock button on the remote fob.the hazards should flash once to confirm.
turn off ignition and remove the key.
unlock the drivers door and open the door.
the remote should now work.
if that does not work,the dealer can reprogram the fob with the obd diagnostic system.

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.

You should have a switch under the dash and then just turn it to the off position. If you don't or it doesn't work (like mine) then, there is 2 fuses under the dash, put the key in the ignition and turn it forward, pull one of the fuses out and then start the car that should reset it. Then you should be able to put the fuse back. Sometimes if you put the fuse back before starting it the alarm will just continue to try and reset itself, but not start the car. If that doesn't work I have a mechanic who can pull the whole alarm out and when hes done it would look like nothing has ever been installed. Hes very good. I ended having him take my alarm out when I had the same incident happen after the 2nd time.