This 1999 Lexus ES300 came in with the service engine soon light on. I did a quick code check and found code P0171 stored. Fuel system lean bank one. I believe in checking simple things first and the Mass Air Flow sensor is very easy to access and remove. One plug and two screws.
Pull it out and look into the holes in the under side.
There was dirt build up (fuzz) on the sensor elements. A light and very gentle scraping with a small screwdriver and the elements were clean again. The elements are deep in the sensor and you can barely see them in the picture below. They are the two gray, horizontal lines, deep in the recess.
Cleared the code and checked the fuel trims and all was well. I like something easy for a change.