My Garmin 945 is prone to showing "CPE expired" even right after a sync, and when it does this, it measures distances very inaccurately. I've tried fully resetting the watch, doing multiple GPS soaks both with and without starting an activity, and syncing many times with Garmin Connect. I've spent many hours on the phone with Garmin customer support, who said things like, "What is CPE?" and claimed this is not a known problem, which doesn't seem true given existing forum posts about this. They sent me a replacement watch, and I'm having the same problem with the new one.
Given how unhelpful customer support has been, I'm wondering if any other users have found their own solutions.