You could search the forums for engine failure or similar terms, read through the threads, and build a list of usernames of people who had a 2.5T replaced. There are two general symptoms that seem to lead to engine replacement: misfiring or a coolant leak. Both seem to be related to issues with the cylinder head. I came up with about 10-15 people when I did this before. The numbers have since increased.
Much like the V6 water pump issue, I seriously doubt Mazda will ever disclose the frequency the problem occurs.
You have to wonder how many people came to this forum only because they had the leaking coolant issue in the first place. This will greatly skew the numbers in one direction. The only real way to get a feel for this is to get a list of CX-9 owners and randomly select a group of them and ask them this question. That's not really gonna happen though. The next best thing would be to get a dealer or a (former) dealer mechanic or Mazda staff to discuss this. Also not likely. I'm gonna go see what's being said over at Consumer Reports. I have a membership. When I bought my CX-9 back in January of this year, there wasn't much chatter of engine failures for any model years from 2016+. Maybe that's changed....
So possibly one report so far on CR (is warning light something people are reporting alogn with the leaks?) Whatever it was it didn't seem to have affected the overall rating and who ever posted that still gave it a rating of 5/5? Or am I misreading that?!?
I think Cr suffers from the same thing we are, low feedback numbers. There could be 5 people that filled out a CR survey for this for all we know. Ugh.
While not exactly the same, Ford had a similar issue with their 1.6L Ecoboost. They used it for 3 years and then replaced it with a completely new 1.5L Ecoboost. It then took another 3 years for enough of those engines to have coolant leaks resulting in engine replacement that there is now a class action lawsuit. The fact that Ford replaced it so quickly, to me says that they knew it was an issue and were just waiting/gambling to see if it was a wide enough spread issue that they’d have to do something. I think that Mazda not completely redesigning the engine and instead just beefing it up a bit bodes well for us as owners. Do they know the coolant issue is a problem? Yes. Do they know what is likely causing it? Probably. And they don’t see it as enough of an issue to scrap the drivetrain and replace it with something else.
I also agree with all of the others that have said this forum is where the vocal minority will be. For the vast majority of CX-9 owners, they will never come join a forum. Others will only find the forums when they have an issue they are trying to diagnose on their own. Non of my friends or family browse car forums for fun. They only do it when there’s a problem.
What we can hope for is that the coolant leak resulting in an engine replacement appears to be more common than it is just because of the location where the data was collected. Mazda has sold 138,473 CX-9’s since 2016. If it was truly a 25% failure rate, there would be 34,618 unhappy customers. The sites that @arock712 posted would have more than a handful of complaints.
So far my 2019 GT with almost 12K miles is doing just fine, but thanks to you folks I'm paying attention.
Regarding forums; they are the first place I visit when seriously considering a new car purchase. I'll lurk on a forum for a specific make and model for a good year before making a decision as I will discover all sorts of issues that the dealer will never tell you and may not even be aware of.
When I bought my CX-9 I was also looking at the Toyota Highlander and Honda Pilot and neither made the cut because of problems I learned about from their owner's forums; the salesmen were either in complete denial or just unaware of the things I brought to their attention. Big thanks to everyone here who shares their experiences with their vehicles- it is a huge benefit.