Ford Repair: 99 Ford Expedition service engine light., oxygen sensors, ford expedition

Question
Eric,
  I have a 99 Ford Expedition 5.4 liter engine.  My service engine light has been on and I am unable to get it to go off. First thing was egr valve. Husband replaced.  Then I got some bad gas I believe because my car started missing after refueling. Now it is reading system too lean bank 1.  Where do I go from here and with the check light ever go off?  thanks for your help Georgina
P.s. has had a full tune up about 1 month ago by dealer. Had new battery also.  

Answer
Hello and thanks for the question...I'll try to help.
The only way the check engine light will go off is for a mechanic to use a scan tool to shut it off.
If the computer system is indicating "system lean on bank 1" then you might want to tak a look at the tune up condition of the engine and also have the oxygen sensors checked out.
Erik