I had the same code you had, PO171, I cleaned MAF while off the car, and repalced the DOWNSTREAM O2 sensor, under the car and the code is gone.
Could be a vacuum leak they are very prone to happen in the PCV vavle hose.
Try replacing the PCV vavle, it is a few hour job, lots of stuff has to come out. Make sure to have an extra gallon around as will lose lots of coolant wheil you remove the thermostat housing.
There are directions for doing on the site somewhee, not sure where.
