I see Matt G of Oakland (under A/C/Heater) and gordyga of Louisville, GA (under Engine) have already posted complaints similar to the following for their 2014 Mazda CX-5s.

Check engine light with Code PO11A. Dealer spent a few hours testing the heating system and related sensors to reach the conclusion (using the diagnosis tree supplied by Mazda) there is a blockage in the heating core. A corroded heater core should not happen on a three-year old car! Ran into the same problem as one of the earlier posters - replacement heating cores are on back order. Dealer offered a solution to replace entire heater box assembly (no back order issue), and though more expensive, I took it and will get the work done next week.

Between this and an earlier rear brake issue that Mazda acknowledged via a service notice, but refused to cover (over 36K), this may be my last Mazda.

The heater core has corroded badly enough to clog, turn on the "check engine light", and prevent effective heating for 20 to 30 minutes after car has turned on. I live in the SF bay area so it is not very cold. The error code is PO11A.

Mazda dealer says I need to replace heater core at cost of $1,500 plus. My research finds that this is a common problem with CX-3s and CX-5s of similar vintage. Further, Mazda has changed formulation of coolant to reduce corrosion. Also, the heater cores are back ordered 3 to 4 months. Its pretty obvious that the problem is Mazda's failure to anticipate and prevent corrosion. A heater core should last for a lot longer than the 5 years, 43.000 miles I've had the car.

I've contacted Mazda and they are "researching" my issue, saying that if they choose to do anything it will be out of the goodness of their hearts because I am out of warranty. I mentioned that because of the back order, I may have to go through a winter without a functioning defrost, which strikes me as a liability issue.