Do you mean that one night the alarm went off but the next night the window was down a bit and the alarm did not go off? Like someone had entered the car or did the window just get left partly open?
If the second one, some alarms will not arm if a window is even partly down. In your system this should not be the case. However, a bad door switch that tells the alarm that the door is not fully closed could do it. The common failure points on that car are the hood and trunk switches. They will bend or even break free and now the system thinks the hood or trunk is open and not arm. The hood switch is under the hood on the inner fender lip on the drivers side. The trunk switch is to the right of the latch in the rear trunk panel.
The hood switch is the more common failure so I would test it first. It is a simple open/closed switch that grounds out when the hood is opened. The easy way to test it is to see if the alarm will arm. If it does not open the hood and disconnect the switch. Try arming it again. If it now arms the switch is bad. Two bolts hold it in. The rear switch works the same way and can be tested easier. If you open the trunk and the light comes on, the switch is working. If you push the switch down and the light goes off it is okay.
There are also switches in the doors but testing them usually takes a scan tool to verify they are operating.
Friday, February 16th, 2018 AT 9:40 PM