Gm55564891

I already posted the problem on my old car forum VxOn, # 9 on
https://www.vauxhallownersnetwork.co.../#post-4082259 .
While the original thermostat housing with thermistor GM55564891 was very expensive $130, I got one
from the www. When I got it I tested the values of the thermistor with hot water. To my surprise it
stayed steady at 15 ohms, this wasn't a thermistor but an ordinary resistor. So I send it back.
Later on I got the opportunity to test a GM55564891 at an official Opel (=Vauxhall) dealer.
To my surprize it measured (also) 15 ohms, the housing looked a bit different but is was an official GM equivalent.
Now I wonder what the value really should be.
Are there owners of an Aveo 2012 1.6L 85KW T300 (F16D4 engine) who would measure their thermistor?