P0171 is fuel system lean in bank 1 (the drivers side of the engine) The bank 1, pos 1, HO2s (the o2 in the manifold) is telling the pcm that there is not enough fuel on that side of the engine. This o2 has been a problem for the 5.3 and 6.0 engines, most likely on a scanner it would show not much if any movement compared to the bank 2 sen 1 ho2s. This would cause your runs rough and fuel mpg drop because the pcm keeps adding fuel to the left bank. Replacing the ho2s has been the fix for this and the plugs will most likely clear from the overfuel cond after a few min of run time. Your 06 has a 3/36 warr. so it shoul be covered anyway.
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