I have an iPass which has been on an even keel for ages.
Recently I've noticed a couple of apparent failures in using it. I went through one "pay by coin" lane and stayed, with the lights stuck on "STOP" and never changing to "GO", until there were cars building up behind me. I went through, and subsequent tolls gave me the blue (i.e. "You're clear") light. But I think I've seen at least one case whether neither yellow nor blue light lit. My iPass is wedged between the front of the dashboard and the front of the windshield, which should register as being in plain sight to the machines that read them.
The first time, when things were stuck on "STOP", I went to the website http://illinoistollway.gov to pay late tolls. Incidentally, the website is a good example of the notorious bad usability of government-run websites; their form expects you to pinpoint the exact intersection where you have failed to pay a toll; I tried and failed to find what they had in the system matching my license plate, and wrote a contact asking for help identifying any tolls not paid.
Is there a battery or transponder problem? How should I be trying to troubleshoot this? Can I exchange my transponder at a Secretary of State office or other location? My best thought now is to try battery replacement, and if I notice another failure to get a blue light or equivalent, look into replacing the transponder.
I went to replace the batteries in my transponder. While it looks like it might be able to hold 2 AA or AAA batteries, it does not seem to have a usual compartment to open to replace batteries, or for that matter any parts that look like they are intended to be opened by the general public.
Also, to my knowledge the transponder has not encountered any particularly rough service; it has of course been exposed to a usual amount of road vibrations, but I would be quite surprised to find out that transponders aren't made adequately to stand up to road vibrations.