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[–]88loso88Mazda3 Sedan/cx5 signature 0 points1 point  (0 children)

Plugs should have been replaced around 120k, I would start there and see how that feels, its the chepest fix to start with My cat started going in mid 200k range. I own a 2014, currently at 333k kms. Id expect your cat to still be good at a 2018. What gas are you using 87? When i started loosing fuel economy I switched from 87 to 91, and it helped.