I have a 03 ls v8, I took off my stock air box and was maping out a route for a custom cold air intake, and then when i reinstalled the old air box and took my ls out for a drive the check engine light went on and the in dash screen is reading "etc engine fail safe mode" and its been running like sh*t does anyone know what this means and how i can fix this?

Disconnect the battery for about 30 minutes or so to reset the air/fuel mixture settings. See if that corrects the problem. Also, make sure you didn't knock the IAT sensor around either.

alirght thanks ill try that. yea so i did plug and unplug the IAT sensor

You reinstalled something incorrectly. As long as you didn't start the car after taking your original stock intake off then you will not need to reset the PCM KAM for its long term fuel trims after putting it back on. Also your 03 LS has the slot-in MAF sensor that has the IATS integrated into it. Most all newer Ford vehicles have the IATS integrated into the MAF and not separate.

I am sure your problem will turn out to be very simple. Probably just the little MAFS O-ring gasket missing or loose hoseclamp. Just double check things and I am sure you will find it.