We had one at the shop last month .. (4th GEN)
he added a weird looking catch can (used parts from his local ACE hardware for the fittings).
he added a 2.5" throttle-body spacer.
he installed K&N Cold Air Intake (and had more ACE hardware elbows and stuff fandangled in somehow).
he had 2 other items I can't remember now that I type this .. but they weren't as 'major' as the 3 above.
his issue: vehicle idles rough (like it has a Cam installed), vehicle has no power.
I was looking over the engine (watching over the tech who was working on it actually to see what the customer installed and I had noticed those above) .. he hooked up the wiTECH fob and couldn't pull any codes.
I told the tech, pull the mods off or tell the customer to put it back to stock and see if it runs better .. there isn't much service can do because of the nasty way the customer hooked up the mods -- and not sure if they all were the cause or if one caused the issues over any of the others.
We showed the Service Manager and he said to swap out the throttle body as he had seen them go bad on the 4th gens .. issues were still there .. so I voiced my concern again about getting rid (at least initially) all the 'jank' mods the customer did .. we could do it, but it would cost X-hours of labor time.
Vehicle didn't come back, so either the customer was angry, or he gave up and has been working on putting it back to stock to see what mod he installed broke his rig