Horn/alarm issues
 

I have a 2004 G35 MT6 and when I try to set my alarm the horn blows constantly and the key fob will not shut it off. I have to use the key fob to unlock and then start the car to get the horn to stop. This happened 4 times yesterday and now this morning I came out to go to work and the car is completely dead, it wouldn't even jump start. I have a new battery so I know that's not the problem...any ideas?

Has your vehicle ever been involved in a flood?
Have you checked your battery cables for any corrosion?
Check your IPDM

http://www.nicoclub.com/FSM/G35/Sedan/2004/sc.pdf

See page SC 9 and follow the troubleshooting steps with your multimeter

If the alarm is randomly going off after you have armed the car, check to make sure your hood switch is secured. Its directly in front of the battery. Mine was missing a bolt the alarm would go off whenever a cat farted down the street.

Did you ever resolve this issue? Mine started doing that this evening. My battery is also pretty new (Duralast Gold) but when I got home tonight, the battery charger showed the battery was at 61%.

Same deal, arm the system with the key fob, horn goes on and stays on. Not constantly beeping but blowing continuously. I could get in and start the car with no issues and the horn would turn off. So simply locked with the key, wife and I had dinner, got in, hit the lock button on the fob while in the car, horn started blowing again. Started the car but it seemed to start slow (but had been starting fine earlier in the day and before we left for dinner. I did go through a big puddle today on the way home earlier but I've driven the car in really heavy rain on road trips for hours without issues.

I have had issues with the Duralast Gold and AGM batteries. They used to be really good but the first AGM in my Honda gave me a ton of problems, swapped it out for the Gold a few years ago, been fine since. My wife's Escape took 3 different ones before we got a good one. And the AGM in my Infiniti lasted not quite two years so swapped that for the Gold in December last year.

There is very, very little online about this issue but 2-3 instances I found always seemed to end with a dead battery.