I've got an aging Mazda 3 2006 2.0l, can't really argue it's served us, and still serves us well. I've had to do very little work to it hence questions coming up just now.

I'm at a bit of a loss where to direct my efforts at the moment, the car generally runs without issue, but for a fair time I've been monitoring the temp gauge - it has a mind of its own!

Sometimes it'll stay at the very cool end of the scale, sometimes it'll go up a quarter, rare occasions just under the middle, but never gets there quickly. Weather here ranges from sub zero to 15-20c most days.

If the cars left running for long enough we get hot air through the fans in the cabin, but the air con has not been cool for a good few years, and the cabin filters in need of a change too. Even when the fans blast out hot air, the temp gauge can be anywhere, but never over the midpoint.

I understand this could either be the thermostat, or the temperature sensor? Weird thing is, no engine codes at all. I've not got the ability to take measurements like the coolant temp via a diag tool so any recommendations of a cheap plugin unit to do that would be grea .(or recommended software, don't want to spend a fortune give the age of the car now).

Biggest concern, in the event the car overheats, I've got not idea if it's happening or not until it's too late.

Where would you start, it worth doing the temp sensor without draining fully first? Then move onto the messy thermostat afterwards?

Any tests I can do to confirm one way or the other?