Bad gas mileage on 2003 Chevrolet Malibu

I have a 2003 chevy malibu. It is getting really bad gas mileage. We have tried and replace multiple parts that could cause this issue. Check engine light has not come on the gives us and idea on what part is not working correctly. Help please

Asked by for the 2003 Chevrolet Malibu
A professional grade scan tool allows you to look at live sensor data. If the coolant temperature switch reads low the computer will enrich the mixture, if Oxygen sensor readings are low the computer will enrich the mixture. Another key sensor that affects is air flow values. Also important to look at fuel trim values long and short term to see what is going on.
Thanks, coolent temp is normal. 198, Oxegen sencsor has been replaced. Varies normal. Short term is -2 to 0, long term -1. Map sensor has been replaced with known good sensor. reading is the same. MAF sensor normal. Cat ver has less than 1 PSI pressure. So..... where do I go now?
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Have you had the catalytic converter checked as if its clogged engine performance and mileage will be affected.
this car does not have a performance issue. Runs great. So I still need help!!
You need to have someone scan it with full computer scanner, not just a code reader. Hopefully the person that is scanning it will know what to look for.
Short term and long term are really not abnormal. Just because the O2 sensor is new doesn't mean it's good. What does the upstream or sensor look like.
What do you consider bad mileage. I have a 3.1 in my car and the best I have seen is intown 18 highway 21.
We bought the car used about 6 months ago. It was doing just fine. Now just to go short distances it is using well over a half of a tank. This is not what it was doing before. If it was doing this from the beginning I would say this is what I am stuck with. But this issue just started.Thanks