your dealer mechanic is an idiot if he suggested you HOBC fuel to a "system too lean code" - System too lean code is triggered when the Long term fuel trims go more than +24%, this condition can be set by dirty MAF or a faulty AFR sensor in the exhaust or even a vacuum leak. A good diagnosis can solve it.
Did you get any working proceeding and result from this dealer? or he just said - iss me haee aaktein daalein - and you said OK.
Never do a battery terminal remove type reset on your car. this is not the 1980s anymore.