Check your Coolant Level (In your Radiator, NOT just the Expansion Tank)

[Lazy2.5]

Open your radiator cap. Coolant should be all the way up to the base of the filler neck.

If it's not, your radiator is low, and you would never be able to tell this going just off your expansion tank alone.

I am currently investigating why I have an air bubble that was not "burped" out of my radiator after recently topping up my expansion tank.

 

[AL Cx5]

On other cars, when the expansion tank fills the radiator with no air bubbles. I would think a Mazda works the same...

Air bubbles can come from a failed head gasket. I sure hope this is not your issue! Another cheap fix I did on my 06 recently, changed the radiator cap. I'll be changing our CX 5 rad cap in 2026. Supper cheap and easy.
MotoRad T16R cap is $8.58 Amazon and $10.80 shipped from RockAuto

 

[Lazy2.5]

On other cars, when the expansion tank fills the radiator with no air bubbles. I would think a Mazda works the same...

Air bubbles can come from a failed head gasket. I sure hope this is not your issue! Another cheap fix I did on my 06 recently, changed the radiator cap. I'll be changing our CX 5 rad cap in 2026. Supper cheap and easy.
MotoRad T16R cap is $8.58 Amazon and $10.80 shipped from RockAuto

I was thinking of changing my rad. cap, but I see nothing wrong with it. the seals are good, spring is good.. when twisting the spring-loaded component inside the cap, I noticed it was gritty. I rinsed it out, cleaned the seals, topped up the coolant and put the cap back on. After the next drive, if that air bubble is still present, i'll have to investigate further. My expansion tank is full.

 

[AL Cx5]

FWIW, my 06 truck Rad cap was 4 years old. ECT was acting weird and losing coolant very slowly. Visually I couldn't see anything wrong with it. I burped the rad several times. Still coolant temp was not consistent. I never saw evidence of coolant leaking. I have digital gauges so it's easy to monitor ECT. It was an OEM rad cap, Cummins. The new cap, Stant, solved all of that.

It's about the lowest cost fix, so I tried it. You're thread reminded me to change the CX 5 cap out. They are on 2 year cycles or about $5/year....

How to Test a Radiator Cap: 5 Easy Steps! - Ask My Auto

 

[N7turbo]

Where does it say to change the Mazda radiator caps every 2 years? I will probably have my original one for the life of me owning it unless it experiences an issue.

 

[theblooms]

They do fail occasionally, but after like 15 or 20 years. Certainly not every 2!

 

[SparkYellow]

I replaced mine for the same reason as AL stated above. The old/factory didn't look deficient, but the new one fits a lot tighter.

 

[Lazy2.5]

OK, if my radiator air bubble reappears, I will replace the cap. 11 years old, 345,xxxKM, original.

 

[SparkYellow]

My original cap was 8.5 years old and 167K miles (269K km). It wasn't exactly responsible for any particular symptoms. I should put it back and monitor for problems.

 

[Lazy2.5]

My sister has a 2014 Mazda 6. There was almost no coolant in the expansion tank when I topped it up today, but the radiator was full.