I assume you meant to say 28.3, not 26.3? I would start with the older update files - try 21.9 or 22.6 and see if that works. Some of the newer updates require the older ones to be installed first.I installed a (2018?) UConnect 4C UAQ in my 2014 GC and am trying to update to the latest software version. I'm getting "files not supported" error when plugging in the USB. I've formatted it FAT32 and am using the 26.3 update files here in a "VP4R_Update" folder on the thumbdrive. Attached is a pic of my Radio Part Information screen. My software version is "100B" which I've not seen in any other screenshots. Application version is USA-R10.8.0_20160317-1000, which seems like a very old software version. Any ideas why it's not working?
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