How can I confirm that the pen type thermometer is still accurate after a period of time or after a drop?

MEDISANA thermometers are equipped with an automatic self-test. Every time you switch on the thermometer, an internal program
performs an accuracy test. If the accuracy is within ±0.1°C, the thermometer will automatically switch into the measuring mode. If the accuracy is outside ±0.1°C the thermometer will show 'ERR' in the display and it is impossible to take a measurement.

As it becomes older, the reference thermistor used for the accuracy test, may change its value. In such rare cases, even the self-test does not recognise it. This is why for security reasons and in accordance with international standards, each thermometer should be controlled every two years against an accurate reference device.