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P0174 and p0171 are lean codes. These codes set when the engine detects too little fuel in the engine compared to the amount of air entering the engine. Most of the time this is caused by an air leak- this is often called a vaccum leak. Air entering the system which cannot be metered by the mass air flow sensor is burned in the engine casing the lean condition.
Another cause could be a weak fuel pump that is not able to provide enough fuel to the engine to keep up with the amount of air which also triggers a lean code.
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