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TBH, after about a year or so of ownership (2006) I noticed an occasional slight rough idle. The dealer never was able to find the cause. However, within the last year or two, it's quite pronounced during summer driving at a red light with the A/C on.
For the last little while though, my Mazda has had a stealth code of P0661. It shows up in an OBD reader but the light never shows up. When I told the Mazda tech about this, he was nice enough to hook up his work laptop to it and it also showed the code, but the CEL never showed up. He says it has to do with the vacuum solenoid.
Additionally, for the last few years, I have been smelling a subtle gasoline like smell from the front driver's area. It happens when the car has been driving typically over 30 minutes only when stopped. I managed to somewhat trace the smell to an area near the driver's wheel well.
With the PCV in our Mazda, when it goes bad there would be engine oil leaks but you might not see the leaks. The service advisor says replacing the PCV is preventative.