Check engine light on my Jetta 2001

I bought a used 2001 Jetta GLS 2L WITH 105000 KM. I had the check engie light on, I brought to my garage and the code read PO171 fuel trim bank 1 system too lean, I had the mechanic reset the computer, I drove for 2 days and the light came back on, after 3-4 days of driving the light went off abd have been drving for 2 days now without it being on.

I know that this can be as simple as a vacum leak just changing the tubing that recirculates the oil fumes or it could be a mass airflow sensor or the o2 sensor?

However I am confused when the light went off?

Should I disregard the problem until the light comes on or can someone tell me exaclty what the problem is. I know I can have a diagnostic run on the engine, however this can take both time and money to find the exact cause.

Help

thanks.