I also can confirm that the P0171 (system running too lean) code was caused by an accumulation of crud on the sensors. On my 2001 Corolla the air mass flow sensor is very easy to remove (2 screws) and is very easy to access. To clean the sensor I used brake cleaner which contains isopropanol, heptane and CO2 as propellant. This cleaner, although it's for brakes, doesn't leave any residue and does a fine job of removing the deposits. In addition to the P0171 code, I noticed also that the car hesitated slightly on acceleration; that hesitation is now gone. The air mass flow sensor as shown in the photos is probably the same for many Toyota models. These photos were posted on a Toyota 4x4 discussion forum, yet the same sensor is used in my Corolla. Before cleaning: After cleaning: General view of Toyota air mass flow meter. As I said, I didn't use contact cleaner, I used brake cleaner.