I have a 2004 FX4 with the 5.3, I just replaced the alternator, pulleys and serpentine belt, installed rancho quicklift loaded/rs9000 rear shocks and got an 18" magnaflow put on. After driving for a few days when I put the truck in park the rpm's drop and some times it dies, it also does this if I put it in neutral and put the parking break on. The display then says check gauges. I'm lost on what the problem could be and need some help. Thanks

 

Try cleaning the intake really well and check the air filter...I had to replace a battery on mine, and it did the same thing to me, I think it has to do with the computer resetting, and having to relearn the system...but a can of throttle body cleaner did the trick for me.

 

You might be using wrong grade of oil,change the oil and clean the throttle body.

 

Pull out and check the iac too, or you could have a bad maf...

 

I think the "5.3" is the problem
I think you meant 5.4. Anyways, Ill let these guys help you with the issue, they are probably more knowledgeable about the subject than me.

 

I realized that I accidentally put 5.3 instead of 5.4, God knows I would never want a 5.3 because that means I would have bought a chevy. I cleaned the throttle body and mass air flow censor and the problem was fixed. My truck actually runs like it did when it was brand new.

 

You mean you were not running the wrong grade oil ??

 

Hey it seems this is pretty common problem, as even i have found it happening with my vehicle as well looking forward for more sloutions.