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Most lean codes are actually caused by either a vacuum leak or intake manifold leak. Other possible causes include intake air tube leak, clogged pcv system, clogged air filter, fuel delivery issure, or internal engine problem.
Start off by checking and replacing as necessary the air filter and pcv valve.
Next, check to make sure air intake tube that runs from air cleaner to throttle body is secured tightly and no cracks or holes in it- replace if necessary. Also make sure air cleaner housing has no cracks and that it is closed securely.
Next, check all vacuum hoses and pcv hose in the engine compartment for cracks, breaks, disconnections, swelling or collapsing. Replace or repair as necessary.
Next, with engine idling, use a water spray bottle and spritz water on each vacuum hose one at a time- if idle changes, replace that line. Next, do the same with throttle body base and intake manifold base- with engine idling spray water around base of throttle body and base of intake manifold. If idle changes, replace intake manifold gasket or throttle body base gasket as necessary.
If all of the above are ok, then the fuel injectors need to be tested with a diagnostic scan tool to ensure that they are both receiving a pulse signal from the computer and that they have the correct fan spray pattern. Next the fuel pressure regulator has to be tested for holding pressure and finally the fuel pump tested for correct pressure.
If all that is ok, then the engine cylinders need to be tested for both compression and compression leakdown, which could indicate a burnt or sticking valve.
Most of the time it will be either a vacuum leak or intake manifold gasket leak. That is why we start there in testing