My guess would be that it's something electrical going wrong for you that is wasting your battery. First, does your alarm work? I am not just talking about the little sound that it makes when closing your doors via the remote, does it actually works after locking your doors? Test it, see if you can hear it going off.... Sounds funny, but as I mentioned before, it could be your hood sensor... If your hood sensor is busted, your alarm will be going off silently without you know.... Also make sure that your drivers side door is locking properly, after locking your door, check and make sure that it's really locked by trying to open it manually without the remote. Again, if your alarm is on silent, it will still make the locking sound, that's why it's best to check if the alarm is working, that was my problem. My brothers problem was the drivers side door was not locking properly, which made the alarm sensitive and every time the wind hit the car hard the alarm would go off.... Also, check to see what setting your alarm is on, if it's on sensitive anything will make it go off, depending on where you live, if you park the car away from where you live, you may not hear the alarm going off... It's annoying, but you really need to check the little things first before changing the very expensive things out only to find out that they were not the problem at all...*

Also, do you have an after market amp, another problem may be that the amp is not turning off when you turn the car off....

Also, do you have an iPod in the car, Another problem can be if you leave the iPod in the car over night, it may not be turning off and believe it or not it can waste your battery badly...*

And my last suggestion will be to make sure that all of your lights within the car is turning off as well.... Since our car has a delay in the lights turning off, we tend to walk away from the car without thinking twice about checking to see if the lights are off all the way, double and triple check just to be on the safe side....

I had the same problem I replaced the FSU... I think it solved the problem. The car did not die or any faulty errors.