How to know what the transmitter of the decom is ended

I have the Dexcom system for two years, but I will have used it in total about 7 months.My question is how to know that the transmitter has come to an end.It gives me the lowest blood glucose than in reality (something that did not happen before) and I get ???com more frequency (but does not lose communication) ... According to Dexcom's information, to know if the transmitter is being exhausted, it has to be given more frequently ???And the scope is lower (live precision and lose a sensor!)