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The common problem on the 2000 Ford E 150 that sets the lean codes 0171 and 0174 is ofter cause by a rotted elbow on the PCV line where it connects to the back of the upper intake. When this happens, it creates an unmetered air leak and this can cause a lean engine misfire. Remove the dog house (engine cover) and follow the PCV line around to it hooks on to the upper intake, I suspect you will find a large vacuum leak. Replace the elbow .