The following may be indications that you're breathalyser sensor needs replacing
If the readings that you are getting with your breathalyser seem inconsistent, it is a sign that the breathalyser sensor needs changing. If in doubt it is better to change the sensor.
If the breathalyser displays Sen, or CAL this indicates a sensor failure and the sensor should be replaced.
If the breathalyser cycles through the countdown, but refuses to sample and give a reading, this indicates the sensor should be replaced.