You can also wait an hour or so after the calibration not accepted to try again. I had that happen a couple of weeks ago. I know it says to wait for 15 minutes but the tech support would say at least an hour

Yes. If it fails within 7 days Medtronic replaces it

Thanks. If the sensor is fairly new do you get a free replacement?

Mine does that sometimes, it ends up with a failed sensor and I have it replaced.