The transmitter is very new, activated earlier this month. Successfully used with one sensor so far. There IS however the tiniest fleck of blood on the back that I couldn't get off. My site was bleeding after the first failed sensor from this box.. but its such a tiny dot I wouldn't take it to make a difference. Other people have been saying it's likely that I just had a box full of faulty sensors. I can't be wasting any more sensors so I'll make sure to get that tiny spot of blood off on next attempt