Few days back my 2003 Mazda MPV started misfiring and OBD code was P0300. First mechanic has replaced the all 6 spark plugs (Bosch) code went away but still there was misfiring. He was unable to identify the root cause of issue. I went 2nd car workshop, took about 4 hrs to diagnose the issue but unable to identify and charged his fees and told to come back again for cylinder leak testing for $180. After that I visited 3rd car workshop with P0300, he replaced all 6 spark plugs and coils. There was little relief but van still vibrates. Now the is P0174 (system too lean). Could anyone please help on this and point me to right direction. It would be greatly appreciated.