The two hour warm up starts when the green light flashes. We have two transmitters, so what we do is that I put the new sensor in while he's sleeping, and connect the second transmitter, leaving the first one still operating. In the morning, we take the old one off, put the new transmitter numberr in, and restart. We get asked for a BG within 5 minutes. No 2 hour warmup! And the new sensor is nice and wet and very accurate from the beginning. Yes, we get the weak signal on day 7, and we change then if we haven't already.