Mitsubishi Repair: engine light, mass air sensor, 2000 mitsubishi mirage

Question
2000 mitsubishi mirage DE,manual
The engine light went on for some days and I got the code: P0170(fuel trim malfunction,bank1), but it went off when I took it to mechanic. I was told that  they could not diagnose it until the light was on.
But, I feel the car lost some power.
could u give me some advice?
Thanks

Answer
Guo,
That code typically comes on due to an air leak, clogged fuel injectors, clogged fuel filter, or even a bad mass air sensor.  I have answered other questions from people with similar problems, and some have had luck simply using a fuel injector cleaner with a full tank of premium gasoline.  I've had good luck with STP Complete Fuel System Cleaner, available at most auto parts stores.  I would also recommend you replace your air filter.

If your light had turned off, there really isn't much any mechanic can do, though you can ask for them to check for any air leaks in your intake system.  But if your light comes on again and stays on, the problem probably is with your mass air sensor, and would probably need to be replaced.
Good luck!