My service engine soon light came on the other day and I had it scanned. They said it was code 172 "bank 1 running rich" they say it is the O2 sensor needing to be replaced. Can anybody give me more feedback as to what this is or may be?

The oxegen sensor tells the powertrain control module the exact oxegen content that is present in the exaust so that it can make adustments to injector-on-time to control fuel/air ratio.

I'm assuming you mean DTC P0172. Faulty O2 sensor (on down pipe rear exhaust) is a good possibility, but not neccessarily the problem. There may be other factors to be considered when the PCM detects a rich condition. Has the O2 sensor ever been replaced? How many miles on vehicle? What is current mpg? MAF sensor or Fuel Pressure Regulator (leaking into vacum line) could be at fault also.

This code translates to the PCM alerting you that it is leaning out the fuel ratio beyond the programs acceptable parameters.

Having fought P0171, the first things you should do are ensure no plug wires are touching the O2 sensor wires.
Then check your battery connections. Any flucutation in power can send the O2 into these fits and cause P0171 and P0172 codes.

Typically we find this is not a bad O2, because the O2 is reading rich and the pcm is leaning the fuel. The PCM is telling you it'* seeing an issue because the O2 is reporting that information.

Donky, let'* also figure out what year and model you have. Can you put that in your signature by editing your profile? (link at the top of the page).

It is a 2002 SSEi with 61K on it. I have never changed the O2 sensor before. I am currently getting around 21 MPG on the highway with a some what heavy foot. Before this started I was getting about 20 MPG.

Those mpg numbers are low for highway, your fuel economy is not good. When the PCM detects an excessively rich condition, it will set DTC P0172. Follow the instructions BillBoost laid out & check those items that he called out. With 61k, it may not be a bad idea to replace the O2 sensor, plugs & wires for general measure.