External temp sensor

Hello.
My 2007 XS diesel showed a temperature reading on the small LCD screen of 0 degrees and the snowflake was on the dash when it was 20 degrees outside. I disconnected the sensor and it showed 10 degrees on the LCD with no sensor fitted and stayed at that.
Thinking it was the sensor I purchased a used one from eBay that was said to work. I installed that but the temperature still reads 10 degrees, (and its a warm day!) Assuming the “new” one is OK can anyone tell me what the problem may be? I have checked the ohms reading on both sensors across the two pins and they both are showing around 31 ohms. Is that a good reading or are they both duff?
Incidentally I have removed the fuse for the Webasto heater to stop that coming on. Could that affect things?

Any help appreciated.

Cheers,

John