So I worked on my Mom's 12 GMC Acadia what a pita. Replaced serpentine belt, idler pulley and tensioner and plugs oh and rear brake pads. I know why gm designs there vehicles a certain way to make more money from people. Their engineer team really sucks. The idler pulley was a pain cause you have to remove/move alternator out of the way to remove the bolt wtf no need for that botl to be that long anyways another reason I'll never own other brands. Only part that was easy was the rear brakes.
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Everything you've listed are typical and common maintenance parts. Well maybe the tensioner and idler aren't common, but they do go bad. Why did you change those? What made you think they were bad? Are you sure those parts were OEM? Not some cheap Chinese product bought before you at Cheap Auto Parts R Us...
You're pointing to GMC as cause. What's mom's driving style... City, highway, jack rabbit starts stops, old granny starts, etc.
There's nothing wrong with checking brakes and pads. Are you sure they were really worn unserviceable? What was the problem for the spark plug replacement?
I find it easier, to change/check these things at specific times... Trying to avoid all the repairs at once. This greatly reduces your frustrations!
Depending on driving style and location...
Front brakes - every 2 year ck
Rear brakes - every 3 yr ck
Serpentine belt - annually ck, prob replace every 2ys. (Keep the old one for those just in case moments.)
My list can keep going if you'd like... Or check the owners manual.
I work on multi million dollar fighter jets (30ys). You'd think they'd never break... If that was the case, I'd not have a job.
Yes, engineers are too blame for some things. I'd challenge you, engineer something better. Over a million parts all working together. We people DEMAND these parts to work, without fail. Yet, when a few fail we blame everyone else.
If man can make it, man can break it - nothing lasts forever...gah