Hello all. Been a member for a long time but recently was forced to medically retire from the military and now have time to search and read on here about mustangs. A short bio of me is I have had 10 or more mustangs in my time ranging from 67 fastbacks to now my 99 gt. I was always deployed over my long career in the military. Hence the reason for lack of participation to this site. But no longer will I be a silent member. Now for my issues with this 99 gt. I recently moved to Puerto Rico from my home state of Florida as to help take care of my mother-n-law and her health issues. When I moved here less than 30 days ago I had the desire for another stang. Was in the market for a fox body though but searched the whole island and all I kept coming up with was crap. So I quickly turned my attention to SN 95's. After roaming the internet and setting up meetings to find a gt that would suit my needs, I came across a 99 chrome yellow gt with the 5 speed ( can't stand auto's), and the one thing that set it apart from the others was the low miles (58k).The body is in excellent shape, sounded nice and the only hiccup to the car was it sat on 20's all around and the car was slammed to the ground causing havoc on camber and rough riding (Fred Flintsone style). But there was potential otherwise. I purchased it on the spot, the ride back was approximately 75 miles, the car had less than a 1/4 tank of fuel. First thing I did was fill up before I headed back home. What I quickly realized was that 75 mile hike back home sucked up 3/4 of a tank of fuel. I thought maybe the sending unit was going out or something was horrifically wrong with the cluster. Knowing these cars with mild upgrades should get roughly 18-22 miles on highway. After getting it home, I quickly went online and purchased from American Muscle a set of AM SR springs for all 4 corners, a set of staggered rims with tires, and a BBK CAI setup. After receiving these items and installing them I began to drive the beast around town, as prior to the new parts, my spine/back could not handle the rough ride the car produced due to the previous owner cut 2 coils off each spring and 20's having no sidewall to take the impact of street imperfections. But after 300 miles or more on the odometer by me, I have filled up 3 times now, and have witnessed the check engine light throwing the same 3 codes out, I have reset the OBDII each time with my scanner adn the same ones keep coming back after about 50 or so miles.. PO420/PO430 and PO455, the first 2 are most likely a result of the previous owner installing a O/R H-pipe with all 4 O2's going crazy without the cats present. The 3rd being a PO455 could be a hand full of things from simpliest being a lose or weathered fuel cap to vent lines or hoses from the EVAP system and so on and on. But being that the O/R H-pipe throwing havoc to the O2's, would that really cause the drastically bad fuel mileage, or is there something else I should be looking at? I do not smell fuel odor in cabin, or do I hear or notice any vacuum leaks or the like. The car idles fine, and throughout the RPM range there is no noticeable short comings in throttle response or power lose. Now that being said, this is my first SN95 4.6, but not my first 4.6, I owned up until I moved a 97 Mark VIII that I tricked out. So, any help or direction would be appreciated with why so poor gas mileage. That being said also, I am not a NOOB as most would say on forums. I have for the past 3 days been reading on this website all different possibilities and reasons, but for alot that I have read, the threads tended to end with opinions being swapped and criticism going knowhere. I am not the smartest mechanic on the block, but I have been around the block a few times in my 45 years. And like to think I am a mechanic with common sense. So any direction would be appreciated in advance. I am contemplating on installing a catted H-Pipe to replace this O/R H-pipe. Or utilize a MIL eliminator system. But would the MIL eliminating set-up work with having no cats at all? Or would I still get the CEL? One more thing, just so you all know what I have under the hood. The engine is in original condition with the exception of the new BBK CAI, stock manifolds, O/R H-Pipe, Flowmasters, all piping is 2.5". No other upgrades to mention to the drivetrain. Thanks again.