I have a 2023 Mazda 3 with 80 thousand KM. My problem started just before Christmas were when I started it to warm up in cold weather I noticed that the temperature gauge never left the blue (cold) zone. When I started driving it would slowly heat up to normal range. When we had a real cold snap -40 C I could drive for 17km and it would barely show it heated up to the next line (1/4). Going down a hill into the valley when the motor wasn't revving you could watch the gauge drop from normal range down to almost cold. I have had it into the dealer and they are trying to tell me all is normal even though I have driven it through 2 winters before and never had this reaction with the gauge. No code light is coming on so they won't even look at the possibility that there is a problem. I have read that others have experienced this also but none have ever said if they found or fixed the problem. Is there another sensor that might cause the problem? Any ideas before I replace the thermostat?

 

If there is no DTC found or your I/M monitors are ready, then the dealership could be correct.
This is one of those that cannot really be diagnosed from DIY and other owners claiming they had the same issue.
If your ECU was scanned and you can verify that with your service information and nothing was found, then I suggest to either get a quality diagnostic scanner that you can directly connect to the OBDII diagnostic connector and look in the 3 basic areas yourself. Then at least you would have better information to post on your thread here.
You can also post up you last service report when you went to Mazda service department for the concern. Or PM it to me to review?
Trust me I am somewhat qualified.

In the meantime, what you could do is get a readily available at most any grocery store is a small cooking temperature gauge. Pace the pointed end in the upper vent. The select upper vent only and max blower speed. Then watch the air temperature vs your dash idiot gauge needle and make some notes. Also post the exact ambient outside temperature registered on the dash as long as your Mazda was not overnight in a garage or shelter. If the green temperature light goes out, then you are now in what is called "closed loop" operations which means that the engine is at normal operating temperature and the ECU adjust all operating based on that and current sensor input. If there is any issue, then you would get a DTC+ engine check light, or it would go into a permanent file and tested at every engine cycle for correction.

 

Welcome to the forum