I don't know when or if it was last replaced.

There is no service engine light.

So does that mean it is functioning at prime operation?

Just looking for ways to help this car run well and increase MPGs

2004 dakota sxt 2wd reg cab 3.7 auto 220,800 miles.

I got the car 500 miles ago.

It might, it might not some of them work for a long time but usually ever 75k miles they should be changed.. Check and see what they look like. If they are all corroded they probably haven't been changed in a long time.

Plug in a scanner and see how fast the voltage at the sensor is fluxuating.If any of the sensors are slow change them.When they get old they are slow to respond and don't correct air fuel ratio quick enough. Just changing them for no reason is a waste of money.