I have a 2008 Dodge Avenger and lost the back to my key and the car started and I drove came back home try to start the car it would not work so I put a new battery in my key and it unlocked the doors but will not start the car what should I do

SOURCE: My Car wont let me start it

Well, my mom found a little button underneath the dash board on the drivers side. And I guess its kind of like the reset button. So she pushed it and started the car and it was fine!

SOURCE: i have a brand new pontiac G5 2008 and it won't start.

battery is in the trunk

SOURCE: 2008 dodge caliber

I just found out that in the 2008 year model the sensor mondular for the keyless remote has a tendency to attract static electicity and that leads to shorting out causing the vehicle not to start unless you disconnect the battery to reset the computer. The tech installed a new sensor module that is supposed to have the flaws corrected. I guess we will see if it does and if this is truely the problem at hand. I do want to comment on my 2008 caliber. I am honestly happy with it.

SOURCE: central locking remote won't open car/car won't start

Sounds like a basic bad battery. Even though its not that old it can be bad....and probably under warranty. Immobilizer or starter will not cause that. Get the bat tested.
Only other thing is called a "parasitic draw". This is when something is drawing current from the battery when its supposed to be off. Usually this causes a totally dead battery, not a weak one like you have.