Gravity CO2 Sensor Operation

I purchased several of these units and they exhibit the same behaviour. I connect them up to 5V and read the analog signal quite successfully. The CO2 readings are consistent with other devices. However, after about 12 hours of operation, the 4 orange lights start a slow blink. The unit appears to continue functioning correctly, still maintaining consistency with other CO2 sensors. Powering down the device no difference. Now that it has started blinking, it does so on every power restart. What does this mean please?