2010 Prius both key fobs dead

I've already scoured the internet, YouTube, and this forum for everything I can find about key fob problems, both generally and specific to Gen 3 Prius but I'm stumped so figured I'd roll the dice and create a new thread here.

Over the last year both original key fobs for my 2010 Prius have failed. They both have good batteries in them reading at least 3 volts. Since I'm the only driver, after the first one died I hung it up and grabbed the spare, then that one started acting up a couple months ago. I pulled it apart to check the voltage, which was fine but I went ahead and installed a new battery anyway, only to have it continue acting up. I pulled it apart and messed around with it some and noticed the LED was not working at first, but after removing and reinserting the battery a few times the LED would blink faintly when I pushed any of the buttons, after which it worked in the car for a few more weeks. Now the LED won't light again and the fob fails to function at all.

I've again verified battery voltage is at least 3vdc. It is slightly lower than 3v when I test against the + side of the battery and any of the various ground points on the circuit board but assume it's passing through at least one micro resistor before getting to any of those test points.

I have very nearly full battery voltage on either side of all the switches. I tried putting 3v from an external source directly to the LED and it glows brightly. I even tried gradually reducing that voltage and still get the LED working all the way down to less than 2V so clearly there is somewhere the voltage is getting interrupted even though it's making it to the buttons. I've cleaned the battery contact points thoroughly and verified sufficient spring tension, which wasn't a problem to begin with since as I said I get full voltage at several test points on the circuit board. I've inspected everything under a lighted magnifying glass and can't detect any defective solder joints.

For what it's worth my 12V aux battery is only 3 yrs old and in good shape but just to be sure I went ahead and put it on a charger overnight to bring it up from about 13V to over 14V.

All of the above notes apply to both key fobs.

At this point I don't know whether I've got a programming issue or a failed chip or other component on the key fobs. If I could find a schematic for these units I might be able to trace the circuit and determine where along the signal path the failure is occurring but everything is so tiny and several traces covered by either the battery clip or the RFID chip that it's really not possible without some kind of road map.

Any thoughts or ideas would be greatly appreciated. I'm probably going to have to spend the money to get new keys programmed for the car but I'm about as stubborn as any DIYer out there so willing to live with situation for a while if there's any chance of figuring it out.

 

Sorry, I should have mentioned that yes, I am able to start the car by putting the key fob against the start button and pressing the brake, so at least the passive RFID is functional. I just can't use the fob to electronically lock or unlock the doors or start the car normally with the key in my pocket.

 

I think you're on to something here. Since I first posted here I ran across yet a YouTube video where a guy was having similar issues after a battery change but then he tried a few different brand new batteries and found that some of the cheaper ones just didn't work. Today being Xmas I can't just run out and pick one up but I do have an adjustable bench DC power supply so I pulled the battery off one of my fobs, connected it to the power supply with a voltmeter in parallel, and gradually brought the voltage up. At 3.0V I still got nothing, but at 3.2V all of a sudden the LED started to blink when I pushed any of the buttons. My bench is too far from the driveway to know whether it's working at the car but first thing tomorrow I'll run to the hardware store and pick up an Energizer or Duracell CR1632. I have to admit that the ones I had plugged into these key fobs earlier were cheap import button cells that came on a big assortment card I bought on Ebay a year ago. I figured since they measured right at 3V they were good enough but evidently that's not the case. Since it appears that these key fobs need MORE than 3V, and the batteries you're directed to install in them are RATED at 3V, it would be nice if Toyota mentioned that you need to make sure the battery is at a minimum 3.2V if you want it to work.