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I'm hooping someone can help and any suggestion is great for me. I have some experience with cars and usually try to figure things out myself and search forums for help rather than go to dealerships. I have 2017 6 sport automatic, I’m the second owner for this car. Here is the problem I’m trying to figure out: after driving for an hour I would expect the lower radiator hose to be hot, just as the top one, but it isn't the case, the lower hose stays cold, while the top one is hot. That makes me believe that there is coolant circulation problem and either the thermostat isn't working properly or there is air in system and /or radiator blockade right? The temperature goes up and stay at 210F firm. Here is what I changed so far:
2 new thermostats, one was bought at AutoZone and second from Mazda. I checked all 3 (with its original taken from the car) putting them in a boil water and all 3 got opened at 180F, so I guess that was not the issue;
New radiator: the original had no leaks but I replaced it anyway as I though the old one was blocked inside that prevent good circulation, but seams it wasn’t as it still no fix;
New radiator cup: I replaced old one because noticed that coolant was overflow at mark of 210F temperature, with the lower radiator house still cold;
I changed coolant completely with running car on and radiator cup opened to avoid any air pockets in a system, radiators fans works fine once it rich max temperature;
I see another part to change as an option left: water pump belt. Again there is no leaks at least visible for me as I monitor it everyday. Any suggestions why there is no circulation through the lower hose would be appreciated.
 

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210F is too hot; these engines run QUITE cool. In fact I don't think I've ever seen mine over 190F when I've looked on my OBD scanner while driving, even in extreme (100F+) temperatures. Find the problem before you wind up warping the head or worse.

Check that stretchy belt. It's also possible (although I don't know how possible given that I've not yet taken apart a water pump on these engines) you have an internal failure in the pump where the shaft is turning but the impeller is not! I HAVE seen that in some other engines and it's a non-obvious failure until you take things apart.
 

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Discussion Starter #3
Let me correct, I applied 210F according to what I see on dashboard, I didn't use OBD but going to do it now, thanks for the tip. I just ordered the stretch belt to replace, will post here with results. Thanks so much tickerguy!
 

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How many miles on your Mazda 6?

A 2017 should still under factory warrenty unless you exceeded the miles for it.
 

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Discussion Starter #5
Unfortunately it's out of warranty, I'm hoping replacing the belt will help in some way
 
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