What does code #P0171 mean for a 96 Villager?

"System too lean (bank1)" O2 sensor is giving this error code. There are two O2 sensors one near the engine and one downstream. It could just be an anomaly or the sensor could be going bad. (Bank 1) is the O2 sensor closest to the engine. I had a PT cruiser that constantly gave this code even after replacing both sensors. If the car seems to run fine, clear the code and see if it reappears immediately. If it does then the O2 sensor is probably bad. If it doesn't come back then it was probably an anomaly. I would get this code every 2 or three months on the PT cruiser and would just clear the code to turn the check engine light off as the car ran fine.

Brian is correct. I would add that a vacuum leak on that bank could be causing the O2 sensor to try and correct the fuel mixture. You can try spraying carb. cleaner around the intake manifold and vacuum lines to see if the engine idle changes. Do it slowly so if the idle does bump up briefly you'll know what area caused the change.