check engine code 0171 help

[Adriaan]

Hi this is the code i'm getting on my yaris. I changed the gas cap for a new one. changed the rubber hoses to new ones. took out the k&n filter, cleaned out the map sensor with contact cleaner, cleaned out the airbox and hoses. I put back the stock paper filter. Not sure what else it could be. Though it was the o2 sensor but my gas mileage is still very good around 615 to 640km per tank. it is only off about 10 to 20 km than usual. another weird thing is every so often the light will go out and the code is gone for a few days then comes back. Any ideas on what else it could be? also changed the pcv valve. the car has 329000km on it. Thanks