Bad mileage

I get bad mileage. The check engine lights not on and it passed a scan… so o2 sensors arr good…sometimesit has a low idle so should I get the fuel pressure checked…I’m thinking filter but its in the tank with the pump…I am in the process of using sea foam to help clean the injectors???

Year, make, model, mileage? And, what do you mean by ‘bad gas mileage’? Change the spark plugs recently?

05 grand prix 3.8 v6 no supercharger… 11 miles per gallon…I plan on sparkplugs tune up etc…as I just bought it

128 k miles…

If you just bought the vehicle you might try replacing the thermostat.

If the thermostat is stuck open the coolant doesn’t get to the proper temperature and the engine uses more fuel.