© DenLorsTools.com Summary: Lean codes P0171 and P0174 are some of the most common codes there are for Ford and Jaguars. If the car runs lean enough there can also be a misfire code. Here’s a recent question from one of our readers. See the original article linked towards the bottom of this page.
Ford Expedition lean codes could caused by a leaky PCV hose.
If a car has a check engine light on and has P0171 and P0174 codes stored, the most likely cause is a vacuum leak. These codes are set by the car’s computer when it sees too much oxygen in the exhaust. Excessive oxygen readings in the exhaust will set lean codes. This does not mean that the oxygen sensors are defective. It is a common mistake for people to replace oxygen sensors when lean codes are present. Just because the code is related to oxygen sensor readings, it doesn’t mean that the oxygen sensors are bad. If both of these codes are stored this means that both banks of the engine have excessive oxygen in the exhaust. If the oxygen sensors are replaced it’s most likely that the same codes will reappear. It is very uncommon for two oxygen sensors to fail at the same time.