'05 AWD Vibe Overheating, then Going back to Middle

[FabioVibez]

I have an 05 AWD, and recently I have been noticing that the temp gauge goes past the middle, then back to the middle, then stays in the middle. This did not happen before, I've owned this car ~3 years

Last week it only went a bit above middle, and then back down, just enough to notice a bit of weirdness.

This week it got pretty much all the way up to the H and then back down to the middle. I especially notice it when going up hills at speed, right after startup.

Other factors to consider:
I have a classic P420 code, and a P0171 that shows up every winter (assuming intake leak, car engine bay is a too rusty for me to DIY it)
It is VERY cold outside, 10-30 degrees Fahrenheit is where I noticed it getting the worst.
My cabin heat takes forever to come on, and sometimes doesn't at all.
my coolant gauge backlight is out, other lights are fine (started happening this spring)

Any thoughts on where I should start?

[SSizler]

Check the coolant level in the radiator when the enine is cold.
The overflow bottle can show full but the radiator can still be low.
If that is OK then I would replace the thermostat.

[FabioVibez]

I filled it up, it took about 1L, when I checked the reservoir it looked below/not showing on the low side. I am a bit concerned since that is a significant coolant loss, but at 184,000 miles I am not sure when it was last topped up.

Is there any common coolant leaks on the vibe? And is that 1L loss considered normal at high mileage.

[andrewclaus]

Check with your local auto parts stores, if they have a tool loaner program, borrow a coolant system pressure tester. Your mileage isn't that high for a Vibe, and that kind of coolant loss is not normal.