Hyundai Repair: check engine light, air flow sensor, oxygen sensor

Question
I have a 2001 Santa Fe Hyundia that has a code P0171 which says System too lean bank 1.  What does that mean?

Answer
Your engine control module (ECM) has what's called a map of expected values for how long to keep the injectors open based on several inputs.  It adjusts these values based on the emissions readings it receives from the oxygen sensor(s).  

There are several basic categories of problems:
1.  air entering engine that is not measured by air flow sensor
2.  air flow reads less air than is actually entering engine
3.  fuel pressure low
4.  defective oxygen sensor, specifically skewed readings.

Case #1 is common, and is often caused by a leaking PCV hose or purge control vacuum hose.  It can also be caused by a leaking or loose bellows between the air flow sensor and throttle body.

Case #2 is also common and is usually corrected by replacing the air flow sensor.

Cases #3 and #4 are rare, but would be resolved by replacing either the fuel pump or front oxygen sensor, respectively.