2005 Ford Taurus bad MPG 18.3 city 19 hwy

First of all, what is the temperature where you are? If it's winter, don't pay any attention to numbers you read. Those are based on ideal conditions that include warm weather.

To see how the engine is running, you need a scanner to view live data. Long and short-term fuel trim numbers will show if the computer is seeing the need to add more fuel beyond it's pre-programmed metering calculations, or if it's trying to cut back on fuel delivery but without success. You can also watch if the front oxygen sensors are switching between "rich" and "lean" properly. A vacuum leak will introduce extra unburned air that will be seen as a lean condition. A leak in the exhaust system ahead of a front oxygen sensor can introduce outside air in between the pulses of exhaust gas flow which will also be seen as a "lean" condition. For both of those conditions the computer will try to compensate by adding more fuel to the metering calculations. Regardless how much fuel it adds, the lean condition will continue to be detected. A true lean condition can cause hesitations and stumbles. You are unlikely to notice a running problem with a rich condition unless it is really bad.

P0171 - System too Lean (Bank 1)
P0174 - System too Lean (Bank 2)

Those codes show both sides of the engine are running lean. That proves the oxygen sensors are doing what they're supposed to do. You don't replace them for reporting a bad condition. The next step is to look at the fuel trim numbers. Most likely they will be high positive meaning the computer is requesting more fuel in an attempt to correct the lean condition. Oxygen sensors only detect oxygen, not fuel, so no matter how much fuel is added, the exhaust is never going to go rich until the cause of the lean condition is repaired.

In this case we can probably rule out a spark-related problem since one misfiring spark plug would send unburned oxygen into the exhaust on just one side. Look for something that would affect both sides like a vacuum leak, or possibly a leak in the fresh air tube between the mass air flow sensor and the throttle body. Any air sneaking in that doesn't go through the mass air flow sensor won't get measured, so the correct amount of fuel won't be calculated.

There are other things too. When any service is done that requires disconnecting the battery, the Engine Computer loses its memory and will have to relearn the characteristics of all the engine sensors by comparing them and various operating conditions to each other. That doesn't take real long, but fuel mileage can improve over a few days. Those fuel trim "lockup tables" have to be rebuilt too. You'll never notice that is happening, and that too shouldn't take too long.

Be aware too that thanks to our wonderful politicians, we have some real miserable winter fuel blends now. The exhaust on today's cars is so clean you can suck on the tail pipe and live to tell about it, but it comes at a cost, and that is greatly-reduced fuel mileage. My 1980 Volare consistently gets 28.3 mpg in the summer but only 19 mpg in the winter. A 1968 Buick Wildcat was so big, you needed binoculars to look in the mirror to see the tail lights, but they easily got 22 mpg. The problem is those cars are dirty as far as emission is concerned. With today's technology and seriously lighter cars, we should be getting 50 - 60 mpg, but the trade-off would be increased emissions.

I wouldn't be too concerned with your fuel mileage yet until it gets warmer I have a friend who specializes in rebuilding smashed one and two-year-old Dodge trucks, mainly dually diesels. His latest project started out getting less than 16 mpg, but the more he drove it, the better it got. Today it's up to over 22 mpg. That's in winter, but not when pulling a big trailer. It took almost a month for the Engine Computer to relearn everything as he drove it.