I have done the thermostat and temp sensor downstream o2 upstream is rounded off I am gonna do it soon and I have done spark plugs and air filter any other thing that could cause? 7 mpg highway or about anything other then the sensor that could be the cause because I am about to do a part order.

 

You already fired the parts cannon so it's a little late but try to figure out what is wrong. For a 96 and up truck you can get a cheap OBDII reader and see what the O2 sensors etc. read and what the fuel trims are.

 

How is the truck equipped? Engine? Transmission? 2 or 4 wheel drive? How are your front tires wearing? When did this start or has it always had bad fuel mileage?

Personally, if there's not a CEL showing codes present, I'd check the alignment and the brakes. If you have a caliper or front flex hose causing the brakes to drag, gas mileage will tank. The same if the parking brake is dragging or a wheel cylinder is stuck. If the tires aren't feathering on one side, like the outside, I'd look at the brakes. It might be as simple as a collapsed flex line in front.

 

Open up your throttle plates and look down your throttle body for oil. If you see oil, replace your intake plenum gasket and spark plugs.