A couple of months ago my Check Engine light started coming on sometimes when I started the car, and going off after a couple of days. I had the OBDII checked and it said PO172 :Fuel Too Rich. I noticed no operational issues with the engine. After a while it stayed on all the time. The car still runs fine, but I am noticing the engine revs abit when it first starts. I get between 20 - 24 MPG. What should I be looking for? Thanks in advance for any advice!

 

Maybe a vaccum leak.
Maybe a bad Mass Flow sensor.
Maybe a bad O2 sensor.