........ Both I and Fred replaced batteries and it still shows ---.
I had one where the new battery would last only a couple of weeks. I replaced it a few times to no avail. I then replaced the sensor and bench tested the faulty sensor over subsequent months. Each new battery I fitted would show 3.0v or less after a month or so. I deduced that the motion sensor within the TPMS sensor that wakes it up each ride was the cause. I tried different brands of battery with same result.I am convinced my failed unit was due to a failure of the motion sensor as well. Except in my case the motion sensor failed in the off position.