Modern Vespa

The thermometer does not work. It says minus 17 degrees, then -16, -15 and so on. Ideas?

Maybe heat from the radiator influencing the sensor???

I assume it is the ambient temperature sensor, correct? +17 and after that change directly to -16 etc? for me sounds as the temp sensor is out and a new one solve this crazy readings

OP said -17, -16, -15, etc.

You guys use the thermometer on the Mp3 as a useful feature?

I suppose its reasonable to expect it should work reliably but I just always expect them to be off by about 10 degrees. I use my phone weather app for accurate weather and the rest is just "do I feel hot or cold".

I think its the "Piaggio Room Temperature Sensor". If the YourBan is anything like the 2016 Mp3 500 I have then the sensor is mounted under the handlebar plastic.

Parts Diagram for 2018 Yourban 300. Make sure to go through the catalog for your exact model-year and version to get the exact right part.
https://www.easyparts.nl/explodedview/Genuine-parts-Piaggio-MP3-300-IE-LT-Yourban-E4-2017-2018-(EMEA)-Electric-wiring-m26973-d200411

Link to the part "Piaggio Room Temperature Sensor".
https://www.easyparts.nl/Room-Temperature-Sensor-p33532443

Alternative brand and solution (check before buying)
https://www.easyparts.nl/Climate-Temperature-sensor-RMS-Piaggio-Vespa-E3-p33532444

Now if your talking about the thermostats and sensors for the engine, radiator, etc then thats a different story and it definitely should be reliable.

https://www.easyparts.nl/Thermostats-and-sensors-c21305682/Piaggio/MP3

You are right, if is not a very very cold day I think the temp sensor is out

And if it's a very cold day, it makes sense that the temperature will increase as the engine transfers some indirect heat to the sensor, right?

My ambient temp sensors in general are plus 3 to 5 C out, but I think is a problem about the place where are, is very easy that the hot from the road foul the sensor.

I use it some times only to say "is more hot that I feel" ha ha ha