Medtronic Guardian 4 not workind

I have tried everything with the Guardian 4 sensor including a loss of two boxes (10 censors) to get my reading correct. I put on a new sensor let it worm up and as soon as it comes on my sugar is ether Hi or low according to the sensor(Gardian4) when I test with my BG machine or use a test strip it is always out by mere than 8MML witch the Medtronic device does not except. it asks me to retest in 2 min. Witch I then do. But then I can not see wat the count is on the Medtronic device. It gives me another reading on the BG machine and the it does not except it and then it asks me to retest in 2hours with I do and then it stil does not except it and after that it says change sensor.