Disclaimer: Excuse my car lingo as I am by no means a mechanic or anything that would allow for me to sound educated when talking about a mechanical problem.

I have a 2000 dodge neon and just recently got a check engine light. It said that it was running too rich and that the o2 sensor was bad. I don't remember the exact code but I know that it was the sensor that is right behind the block on the top of the exhaust. I changed the spark plugs out only to find that they were damp and black. So I know for sure that it is running rich. What started this whole venture was that fact that my car was getting about 15 miles per gallon when I know that it should be getting closer to 30mpg.

So my question is, is it more likely the o2 sensor that is causing my car to run too rich thus causing bad gas mileage or is is possible that the plug wires are bad and I'm not getting efficient spark and it it is running too rich? Thanks.

Things to be considered:

I've just changed the plugs out with new NGKs.
The air cleaner was checked and was clean(also recently replaced)
I drive very conservatively to drive to conserve my gas.
The plug wires haven't been replaced to my knowledge but as far as I can tell there is no misfiring.

 

Well if the o2 sensor is messed up and causing it to run rich, that will kill your gas mileage right there.

Other factors leading to MPG reduction:

Misaligned wheels
Improper fuel/air ratio
Under-inflated tires.

Just a few of the quick fixes for bad MPG, it is by no means the only cause.

 

I just got a tire alignment in the fall and got new tires, tire rod ends(tie rod ends? not sure how to say it?).

The o2 sensor can cause improper fuel/air ratio correct? Also something else, the engine kind of lugs behind at acceleration when I take off and then catches. Doesn't seem like he power out of the hole is what it should be.

Something else that should be noted, the car has 140000 miles on it. I bought it used from an auction 2 years ago. Only problem I've ever had was the bad tie rod end and the cam shaft sensor of which I replaced myself.

How hard is it to replace the o2 sensor? I have an extra parts car an I am thinking about doing it myself.

 

If your plugs were fouled like that, any unburned mixture, especially gas (rich-condition), is going to head out of the engine via the exhaust and it will coat and can damage the O2 sensor. A damage/fouled O2 sensor will cause the computer to change all sorts of things, and the most noticeable things would be loss of gas mileage and power.

To replace it, you can use a wrench or go buy the special socket that fits over the wires. Unscrew it and screw in the new one.

 

Got it all figured out guys! The o2 sensor solved all my problems! My car is getting the best mileage its ever gotten!

 

My O2 sensor is on the way out as well. I was watching it on a scan tool. It was switching VERRRRYYYY slowly. Damn lazy O2.