The replace sensor error usually happens when the filament that goes into the skin get loos in the skin or comes out of the skin. That is usually caused by movement of the sensor like from the cat scratching the sensor or applying it in a location excessive movement like over a bony area.
The second problem of same reading I have not seen that discussed before.
You can try calling Abbot and they should send you a nen sensor especially for the second problem. Just do not tell them it is for a cat. When I had to talk to meter companies I say I am a caretaker and never say what a am taking care of.