got a check engine light come on today.

code reader said bank # 1 running rich.

Any ideas as to the most likely cause(s)?

Im still suspecting a bad o2 sensor. Im getting decent power and the motor runs good (tho idle is a tad rough) but I get horrible gas milage.

 

You only have "one" bank with an I5 engine so it's telling you the engine is running rich.
Was that the only code ??
Punch in the "P" code and 850 into google, ask.com or whatever search engine you like to use and you should get a bunch of information on cause and cure.
If you haven't tossed in a fuel injection cleaner I'd give that a try. It's cheap and it's more like routine maintenance. Nice to do each oil change IMHO.

Second, take vacuum line off the fuel pressure regulator and see or smell if there is any gas in the line or coming from the nipple on the regulator. There should be no gas present. If the diaphragm in the regulator starts to leak the engines vacuum will suck raw gas into the engine driving it rich.

 

Thanx for the tip on searching the code/850....hadnt thought of that.
Yes, that was the only code. I earased the code...then drove about 10 miles and the light came back on...same code.
I will check the FPR as you mentioned.

I am averaging 12mpg...granted I have been driving mostly in-town and have a heavy foot But it would be nice to get alittle more than 12mpg

I did throw in fuel injector cleaning with my last tank, and put Z-maxx in the motor with the oil change.

I will check the FPR, and also I will do the o2 test I read about on ipd's website.

 

+1 to what kFrog said.

In addition you should ck the fuel pressure at the rail; FPR could be bad (even w/o the diaphragm leaking) and cause excessive pressure, making mixture rich, etc..

 

The fuel pressure regulator itself shouldn't (shouldn't) be able to bump up the pressure that much to cause this problem. I'm running a different regulator that is 13psi over stock and my mileage didn't suffer. I did it for when I up the turbo and injectors (blues!!).
If the pressure rises the computer should just trim down the injector pulse to maintain the proper balance.
BUT ...... checking the pressure wouldn't be a bad idea if you can get your hands on a gauge without having to buy it
I know the stock pump in an 850 can go up around 80-90psi as that is its rated maximum. I just haven't seen a regulator do that. Usually they go bad by getting weak and dropping pressure below the operational minimum causing a stumble or higher speed lack of power and finally when they really go bad a no start.