I own a 2007 Mazda 3 with a thermostat history. On and off over the years my check engine light will come on and I'll have it checked and always get the P0128 code which means a faulty thermostat. In the past it has always just been stuck open.

ANYHOW. If the thing is in fact stuck closed in any particular instance, and car begins to over heat, will my temperature gauge even tell me the correct temp? I don't know if these two things are connected to one another somehow and one can cause the other to malfunction.

Im sure this is a very stupid question and I am probably overthinking it.

Feel free to rip me apart. And thanks!