Today, I was driving my mum's car, a Citroen C3 1.4 HDi from 2004, and I noticed that the coolant temperature was rising very slowly. Just as a reference, this is what the dashboard looked like (picture from different car, but for my purposes here this is not an issue; coolant temperature reading was exactly the same as in the picture below):
It actually never went up after one block, even after driving a significant distance. If this coolant temperature indicator is designed like all the others I've ever seen, you'd expect the indicator to be in the middle (i.e. three blocks out of six, approximately).
This would lead me to think that the car did not reach proper operating temperature and that the thermostat is most likely malfunctioning and stuck in the open position. However, she said that she mentioned this exact observation to a mechanic and that it's "not an issue" and that it's "normal for the indicator to not rise after one block", so she doesn't believe me. This, however, makes no sense to me at all. If all of this were true, there would be no way to know anything about the coolant temperature until it is at operating temperature, yet there are five blocks left to indicate overheating (out of which only two are red). This just doesn't make any sense to me.
I wanted to use an OBD2 scanner to get the temperature reading, but this car has a non-standard connector so I couldn't do that (I can of course buy the right part but that might be a bit overkill for something like that). Maybe I could use probes to measure the temperature, but I don't really like working near potentially hot coolant.
So, actually, the core question is very specific: is one bar out of six indicating operating temperature in this car? Again, I highly suspect that this is not the case, but since a mechanic told her otherwise, I'd like to get confirmation.