00' Engine light comes on... "System too lean?"

Has any one experienced this problem yet? I have a 2000 Toyota 4 runner. I recenty came back from southern california from olympia, wa and on my way home, my engine light came on. I read my manuel and it said usually when you dont tighten your gas cap until it clicks, the engine light will come on. I took it to auto zone to have them check out this problem. They did a diagnostic test on my 4 runner and it read back "system too lean". The guy thought maybe i need to put some fuel cleaner in there, maybe it was from the california gas?!? I put a bottle of fuel cleaner in my gas tank but 3 weeks later the engine light came back on and read the same thing. Can anyone give me some suggestions?

Welcome to the AF!

Could be a hole or crack or loose connection in your air induction system
Clean your MAF meter and connection with electronic cleaner
Check your engine coolant temperature sensor
Check your HO2 or A/F ratio sensor and circuit (Sensor 1 Bank 1 - the one in front of your cat converter.
Check your fuel pressure

Start with the top one and work down.

I had to take mine into the dealership for this & a pedal position sensor problem -- Cost around $540.00 to fix -- but I have over 120,000 miles & this is the first time to have any work besides maintenance.