The ECM is doing exactly what its supposed to. Telling you something is wrong with the engine. At least in its opinion. Everything is programed to have those O2 sensors give feed back to the ECM so it regulates the fuel injection correctly, adjusts the timing, and burns off the bad stuff in the CATS. Once you disconnect them, now the ECM goes "Houston we have a problem" and lets you know by turning on the warning lite. With the O2 disconnected it dumps more fuel than needed, so mileage goes down. Also because you are now putting more "raw fuel" into the cylinders it dont all burn. So you get unburned gas in the Cats, so they run hotter to burn off the unburned fuel. If you punch out the CATs it kind of goes the other way. It leans out, which makes the Cylinders run hot because some of what the fuel does is cool the cylinders.
2013 XCT added Jan 2015
pull back bars, passenger arm rests, HID Low beam, Silverstar Ultra headlight high beam,Drag Specialties Pillow Top seat with driver backrest. Glasspack mod Mufflers.
2009 Royal Star Venture-Traded Jan 2016
Trailers are for when it's broke aint it?