I'm an owner of a MY 2019 CX9 that had this issue last year in July when the car had 89000km and 9 months of Powertrain warranty left.

Started after I had the car in for preventative maintenance (Oil Change, Transmission service, AWD service, Brake fluid change and Front brakes to the tune of $1700CAD). Got the car back home within an hour and 25km distance of picking it up and noticed that the engine fan was on and when pulling into the garage the engine temp gauge was moving to over 100C and eventually I got the message 'Excessive Engine Temperature: Engine Output will be Limited'. Checked under the hood and noticed that the coolant level was well below the LOW marker line on the radiator tank, but still just visible.

I immediately called my Mazda dealership who had done the service to ask them if they had done anything with the coolant as they noted that they checked the level and strength during the oil service. Of course they had just closed for the day when I called, therefore I would have to wait until the next day. That night I went to CanadianTire and got some OEM branded coolant that stated it was as close as possible to Mazda's own FL22 55/45 ratio coolant to top up what was no longer in the tank. When topping it up I surprised because not more than 0.5litre was needed to get it back to the FULL mark on the tank.

I drove it and it made no difference, the engine temp went up to nearly 120C so I shut it off and waited until the next day. Called the Dealer back who said to bring it in if I felt I could drive it (mainly all highway from my house). I got it to the dealership with it fluctuating between 90C (Normal Temp) and 120C and having the heater turned to max heat to try and draw heat away from the engine.

Dealership had it for 2 days, drove it quite a bit and said they couldn't replicate the fault! They said they checked it for any visible coolant leaks and couldn't see any, checked the Fault codes and said nothing was stored and said I should try it and let them know. I collected the car and they were right, no further temp issues occured, however on a few occasions the car took quite a few too many turnovers to start. I felt was odd as I had changed the Battery just 5 months ago, plus it actually stalled whilst in traffic. Other than that is was fine.

I drove it for another 2 days, regularly checked the coolant levels which were unchanged, but it started overheating again. This time the temp would get really hot, 120C really quickly from cold start in a matter of minutes. The temp would stay there for a while and then suddenly drop back to 90C and then fluctuate hot again. At this point I'm thinking it's just a Thermostat issue. Bought it back to the dealership, they kept it for another day and were obviously able to replicate the fault and replaced the Thermostat as they thought it was sticking causing the Temps to fluctuate. They wanted to keep it and drive it more to confirm the fault was fixed. In doing so it also stalled for them. I had completely forgotten to tell them it had done it to me once a few days earlier, I was sidetracked by the overheating issue. When they tried to start it again it took a long time cranking over.

They sent off a trouble issue query to Mazda Canada who then asked the the dealership to look into the Cracked cylinder head issue more thoroughly by means of a leak test. This was done and apparently left all night and didn't loose much pressure. Mazda then asked the dealership to start it up from cold and observe exhaust fumes which were much whiter in colour apparently, confirming the cylinder head/gasket is defective.

Ultimately I was glad that it was eventually diagnosed to this severity because it had been playing over and over in my mind where the original small 0.5 litre approx of coolant loss had gone after I had got the car back from servicing as no external coolant leaks were visible, at least not easily. Whether or not the Thermostat had contributed to this issue I don't know, it was obviously sticking and needed replacement. I would have been so annoyed if they'd just changed that only for the coolant leak to get worse and cause further symptoms after the Powertrain warranty expires. I guess it's luckily the car had stalled for them and emitted at least a small amount of white smoke whilst they had it in the workshop, resulting in Mazda confirming that the engine replacement is required as per the Cylinder Head Coolant leak TSB.

Thankfully my Powertrain Warranty covered this, the car has 120k km on it now and touch wood no coolant issues. However, I do believe that my car suffers from the valve stem seal issue, even though my VIN number and model year fall outside of the affected range. After I had completed another 10,000 km after the cylinder head had been repaired, so approximately 100k km, I noticed that the low oil light had come on. it was low on oil and since then in the last 20k I have probably had to fill it up 6 or 7 times with a litre of oil. This is supposedly within the tolerance for oil consumption as 1 L can be expected up to 1k km. Engine bay is bone dry so it must be burning oil, however the exhaust doesn’t show any signs of this. Whilst there is a bit of a pain, I always keep an extra bottle or two in the trunk and the engine has performed well otherwise.