For the last two days, the alarm after being shut off, keeps beeping. It beeps 6x then after a couple of minutes beeps 6x again. We have checked the module and it is fine. Do we need to change the battery on the second door sensor? If we do, how do we do this. We don't know how to open the sensor. We have had the alarm for approximately 20 years and this is the first time we have run into this problem. Please let us know how to fix this, it is very annoying, especially in the middle of the night. Linda & Steve Douglas