We have 3 SunPasses in the family. ALL work 100%. I have the original model with the small display window (& lights) which will tell ya when the battery is low, uses the AAA type. Wifes & daughters are the newer ones with the 3 triangle suction cup types. Those use button batteries, (round ones) and can be bought at any radio shack. With good batteries, never had a failure.. Try changing the batt..

We also put them just below the tint,front window, behind the R.V. mirror. If it continues to fail after new batt, then probably the transponder is defective, will need to buy a new one, only 25 as you know, cancel the old & xferr funds if any to the new one. Have you dropped yours lately?
PS: sometimes the problem is not the transponder, it is the reading device you go thru, that is why it is important to let them know what vehicles, cause they DO snap your picture AND they will send you a ticket..
Go here and register, it is easy and you can monitor account AND make changes etc etc..
Welcome to SunPass.com I have 2 accounts setup with them, one for my & daughters device, and separate acc for the wife..