2007 Impala P0172 code solutions?

I have a 2007 Impala with about 53,000 miles on it. A month or so ago, I noticed that when I turned on the AC/fan in the car, I'd get a musty smell coming out of the vents. Eventually I got a check engine light that came on. I replaced the air filter, thinking it was probably dirty and causing the smell and hoping it would fix my check engine light. After I replaced the air filter, the smell went away (and has stayed away), and the check engine light went off for a few days. The light then came back and I hooked up friends device to read the code and it came back with P0172 (System too rich - Bank 1). We cleared the code and of course it came back a day later. I checked the air filter and took off the Mass Air Sensor to inspect it and put everything back together and still have the code.

Not really sure if the smell issue has anything to do with this issue, but I thought I'd mention it.

My questions are if anyone has any ideas of what this might be? I'm thinking it could be the MAF sensor, since when I replaced the air filter the light went off for a day or so. Logic would dictate that I'm in the right area. How do I tell if the MAF/sensor needs cleaning and how do I clean it? I don't want to replace it unless I know that is the issue. Am I on the right track or does anyone else have a suggestion? Thanks a lot!

Dirty maf, low fuel pressure, a vacuum leak, air entering the intake duct unmetered, dirty injectors are all possible with this code, or less likely a bad o2 sensor, a comprehensive scan should be done to eliminate guess work.