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Q:

O2 Sensor and poor MPG's on 2001 GMC Sierra 2500

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I have a 2001 GMC 1500 4.8l and I am getting 11-12 mpg's on the highway, could this be an 02 sensor? My air filter is new, plugs, wires, and fuel filter are also new.
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If you had an oxygen sensor causing that much of a fuel economy loss, your OBDII check engine light would come on based on a "lean" fuel trim issue.

A good diagnostic test is in order to see what is going on with your truck. A solid fuel economy test should be done. Make sure there is nothing causing an issue aerodynamically.
There is a check engine light on, I will shoot up to Autozone and have them check the code.

Thanks Dave!
OK, yes, check the check engine light - and if you have P0171 and P0174 together, you have a vacuum leak, air intake leak or something like that...which will make the engine ask for more fuel.
This too can consume more fuel, if fuel is unburned by a misfiring cylinder.

Or, worse yet, you could have a fuel injector on the misfiring cylinder sticking open. Check your oil level - does it smell like fuel at all? Is it high at all?

These are just thoughts. A good diagnosis is in order, otherwise it will be lots of guesswork. Good luck.