i understand your concerns.
i mean, im one of those types who , when i had my qx ( 9 years --never had any issues with it-- daily driver) i would take it to infiniti to get the oil changed, so yeah i would take it to infiniti always because 1, i get a paper trail of all the maintenance done, and 2, well the company made the car so they would know best how to fix it, especially since they work on the same brands all day.
but the reality is , we dont live in a perfect world, and you can go to a dealer and they can have incompetent staff ( ive read the horror stories on these forums ) screw up things, or half do something that requires a return visit...
but in my opinion, replacing cats/exhaust systems are not that specialized that you need to go to the dealer, and if you go to a reputable exhaust shop, where they also do the same thing everyday, you are in good hands there as well.
also lest we forget, forums are a magnet for people who have issues. normally you dont see people sign up on a forum just to tell you how great their experience was at this place or that, or how great their car is - they just drive and move on about their day. its easy to think there are more issues with something that not, when all you see are people complaining or searching for something that broke.
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