Ok first off i will say that i started to have this problem after installing some headers and removing the o2 sensor well lets just say i wasnt the most gentle guy with the o2 removal task. anyways so i thought it had to be the o2... CEL was on so i took it to autozone and i got the infamous P0455 EVAP Emissions Leak (Large). Yes i did check my gas cap but most know that the cap is probably not the problem with our cars only if your lucky. ok so i tell the guy at autozone about the o2 and it possible being the problem for my low MPG but he replied that it would have showed up on the scan. So could it still be the o2? What are all the possible causes for this low MPG problem and out of all what is more likely the cause? If you need more information please do let me know. Thanks guys hope we can figure this out.

It most likely is the o2 sensor. I'm not sure about the large evap leak. I had that code once but it was because the gas cap was bad. Guess I got lucky. I have a 442 code now (small evap leak) but it doesn't affect my mpg. Good luck!

After you installed a header and removed your Oxygen sensor... your ECU runs the engine richer, aka limp mode. You will burn more fuel than necessary. If you still have a catalytic converter it is likely ineffective at this point or on its way out depending on how long you have been running this way. When I worked as a tech years back often times a P0130 through P0140 will soon be followed by a P0420 - Catalyst inefficient due to being overworked from processing so much raw fuel.

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2012 2500HD LML

ok so how can i fix this so called "limp mode"

You put the O2 sensor back on.

oh i should have reworded i meant while removing the o2 sensor i was kinda rough with it but i did reinstall it to the header and its not throwing a code so idk if it needs replacing. Can the o2 sensor still be messed up in some way where there wouldnt be a code being shown