My horn wont sound on my 2007 Ford Taurus. When I hit the alarm button on my key pad the lights flash but the horn doesnt sound

It appears to be a well documented issue!

https://www.google.com/search?q=2007+sahara+unlimited+horn+relay+location

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To arm content theft alarm.
1. Open the door.
2. Lock the door with the power door lock switch or
the remote keyless entry transmitter. The
SECURITY light should flash.
3. Close all doors. The SECURITY light should go off
after approximately 30 seconds. The alarm is not
armed until the SECURITY light goes off.

If a locked door is opened without the key or the remote keyless entry transmitter, the alarm will go off. The headlamps and parking lamps will flash for two minutes, and the horn will sound for 30 seconds, then will turn off to save the battery power.

Every car brand and model has its procedure to stop the flashing light and alarm sounding.
But still the common procedure is Make sure boot and bonnet are closed properly.

It can also be a switch, but check your alarm by locking yourself inside the car, and then press the close button on the key 2 times repeatedly until the lock control on the door doesn't make anything. Then open the door by pushing the lever or open the hood . If the horn sounds and the lights blink your alarm is ok , if only the lights blink you should take it to dealer. You can stop the hazard lights and horn sound by pressing the unlock button on the key.

I would return it to the installer

My 2012 Hyundai Elantra Wont Start - CarGurus

www.cargurus.com > Questions > Hyundai > Elantra

CarGurusJul 30, 2013 - My 2012 hyundai Elantra GLS would not start this morning. ... I hit my door unlock on mt Key fab and the doors unlocked fromt here I proceeded start my car.... it and it wont turn over just makes a light click. not like a battery issue. .... But if Istart from the zero position on the starter it doesn't start on the first try ...

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www.gencoupe.com/.../244433-no-horn-no-lights-when-i-lock-my-car.h...

Mar 30, 2014 - 16 posts - ‎8 authors
I do not know what could have caused this but when i lock my car... ... lock yourdoors with your key fob(might have to hit the lock button twice) ... The alarm/horn should start beeping. ... 2010 Hyundai Genesis Coupe 2.0T R-spec in Karussel White.... My lights flash when I lock it, but the horn doesn't sound.

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www.carcomplaints.com/.../Elantra/.../engine_wont_turnover_wont_start...

The 2013 Hyundai Elantra has 19 complaints for engine won't turnover, won't ...

Your horn relay may be shorting out, making your horn operate all the time. Have a look at your relays. You might find the horn relay is the same as another relay, If it is, try swapping relays with a known good one, and see if the horn operates normally. If so, get a new relay. Good-Day!

it sounds like security start relay malfunction ,check and or change out relay.

This sounds like a bad alarm module, the doors unlock but the alarm doesnt shut down, so it wont let you start it because its not shutting down, the light flash and door unlock put starter disable and horn stay active, most good alarms have liftime module warranties, if you take it back to who installed it and ask them or if it was already in the car when you bought it then you will need to disable or replace it. hope this helps, also to disable it just trun key on then open a open try key off and within 10 sec push the button under the dash its a small red button this will put it in vallet mode.

hit ya boyfreind

if the lights flash when you hit the panic button but the horn does not beep, then the alarm horn is bad. Vehicle has a second horn just for the alarm

I have the same problem on my Chateau, Can't buy new module, probably cost a fortune anyway. Disconnect the Grey connector on the alarm control module. I left the black on on for no special reason. Your power door locks will no longer work. (I'm not sure about the horn. If it doesn't I'll do something else)
The black control module is located on side of the van about 1 foot behind the drivers seat. Remove 2 plastic push in clips and yank the panel away to gain access to the module.