For me signs that a sensor is dying are either repeated periods of ???, calibrations that are more than 2 mmol/L off consistently, or persistently low results that won’t correct after a few calibrations. Or, quite recently, I had a sensor show a flatline at a perfect 5.8 mmol/L for hours when my blood sugar was actually rising and falling, so that’s also another sign.
During those failing sensor times I’ll calibrate three or four times a day, do the three-calibrations-spaced-15-minutes thing, and then restart the sensor if the above two don’t work. Repeat that about four times. If after the fourth time (usually several days to a week later) the sensor is still super wonky and unreliable, that’s when I change it.