I have a 2000 Neon that I purchased form a friend. It has been sitting in a driveway for the past three years and had 15,000 miles on it when I purchased it a few months ago. I now have 18,000 on it.

It idles rough - I have changed plugs, run several tanks of gas through with fuel system cleaner, including "sea foam" stabilizer and cleaner. I have cleaned out the intake manifold as well.

I have started getting an intermittent P0172 engine code, typically after a relatively long drive in hot weather (not that I have had it long enought to see how it does in the cold weather!). The P0172 indicates a rich condition. I have also noted some carbon deposits around my tail pipe.

I have checked vacuum hoses as best as I can - everything looks great. I have found a forum that tells me to check the Mass Air Sensor - I don't think I have one?

Obviously the dealership is less than helpful unless I want to lay out some money to have them scan the system.

I get relatively good gas mileage, seems to average around 25 miles to the gallon, but I think I should be getting better?

I am leaning towards an oxygen sensor, and found I can get both online for about $100 total as an OEM version, not the generic aftermarket.

Am I on the right track? Or does someone have a better suggestion?

Thanks in advance!