DTC Decoded: P0171 & P0174

P0171 – Lean code cylinder bank 1 on right side (passenger) of a V6 or V8 with rear-wheel drive

P0174 – Lean code cylinder bank 2 on left side (driver) of a transverse mounted V6 with front-wheel drive. Since a four-cylinder has no bank 2, this is not applicable to that engine.

These Ford codes are seen when the PCM detects there is too much air in the air/fuel mixture resulting in a mixture that is too lean.

EGR System



  • Service Engine Soon Light illuminates
  • Rough idle
  • Hesitation upon acceleration
  • Poor engine performance


  • MAF sensor is dirty
  • Vacuum leak
  • Low fuel volume delivery
  • Bad DPFE sensor


When you see a lean code on your Ford, it means that the fuel system is not getting enough fuel to the engine, or that there is too much air in the fuel going to the engine. If the condition is bad enough, you will see it in misfiring, idling rough, acceleration hesitation and just bad overall engine performance.

What the Tech Says

When you have a vacuum leak, an airflow sensor that is dirty, or a malfunctioning EGR valve, you get an engine that does not perform, as it should. Here are some of the common causes of the code(s).

Should the problem be that there is not enough fuel, you may have a fuel pump that is weak, a fuel filter that is restricted, dirty fuel injectors or a leak in the fuel pressure regulator.

Dirty MAF sensor – This is the most common cause of the code. The mass airflow sensor (MAF) is found just ahead of the throttle body in the air inlet tube. In order for it to operate properly, it needs to be protected. The air filter is supposed to offer a tight seal that protects the MAF, but sometimes that seal is not tight enough, and the dirt sticks to the sensor wire, causing a coating that prevents the MAF from giving an accurate reading. Fuel vapors can also coat the sensor wire, which can result in an under reporting of airflow. When the MAF under reports the airflow, the PCM does not add enough fuel, which gives it too much air and can trigger the code.

In order to do a diagnostic on the MAF, you need to use a scan tool, select the PID data menu and while the engine is running, read the fuel trim values. If it is dirty, the fuel trim will remain normal (within 3 to 5) at an idle, but when you reach 2500 RPMs, the trim will go up 5 or more into a positive range.

If the sensor is dirty, just clean or replace it. An electronics cleaner can be used to spray the element of the sensor. Do not use anything other than an electronics cleaner, or you may damage it.

  1. Disconnect the air inlet tube
  2. Spray the cleaner on the wire element by way of spraying through the protective screen
  3. Wait several minutes while the cleaner soaks the element
  4. Spray it again
  5. Wait five minutes or so
  6. Reconnect the air inlet tube
  7. Start the engine

If the code returns, replace the MAF sensor as long as you do not have a fuel delivery issue or vacuum leak.

Vacuum leaks – This is also a common cause of the code in the Ford. These usually happen down from the throttle body, which includes the body gasket, vacuum hoses and intake manifold gasket.

Use a diagnostic tool to scan for a leak.

  1. Plug in the tool
  2. Start the engine
  3. Select PID data menu
  4. Check the fuel trim values when idling – a vacuum leak will register 5 or more at idle
  5. Take the engine to 2500 RPM
  6. Check the values for more of a normal range within 3 or 4, but closer to zero

Port gaskets and isolator bolt assemblies on the upper plenum of the 3.8L Fords that have a split-plenum intake manifold can deteriorate, which can cause a leak. This is generally a result of the PCV system allowing oil to be sucked into the intake manifold. The vacuum hose connecting the intake manifold and the fuel pressure regulator can also leak due to swelling.

Steps to Fix

  1. Take off the upper manifold plenum
  2. Change the gaskets and bolts
  3. Put a new front valve cover that will reduce the oil vapor
  4. Ensure that the fuel pressure regulator hose is good; replace it if it is not
  5. Reflash the PCM to make it less sensitive to conditions with lean fuel

Low fuel volume delivery – If the engine is not getting enough fuel, you will see these codes. It could be a weak pump, fuel filter restriction, low voltage or a leak in the pressure regulator.

Use a diagnostic scan tool to determine if you have fuel delivery problems.

  1. Hook up the tool
  2. Select the PID data menu
  3. Check the fuel pressure PID
  4. If the pressure is reading less than specs, there may be a problem at the pump or the pump’s wiring circuit.
  5. Start the engine
  6. Check the fuel trim values
  7. A normal or slightly positive trim at idle and a positive increase as the RPMs increase are indicative of a fuel delivery problem.

If there is no change in the trim values, then you can rule out low fuel volume.

Since dirty injectors have the same effect on the engine as a fuel pump that is weak, you will want to check these as well.

Clean your fuel injectors if they are dirty. Repair or replace any fuel delivery problem that you find.

Bad DPFE sensor - A faulty EGR differential pressure sensor can cause these codes. They have a lifespan of about 60,000 miles or five or six years.

The DPFE is attached by two rubber hoses on the engine to the tube that sends the exhaust gas to the EGR. The sensor has a 3-inch aluminum housing. The inside of the sensor can become corroded, which will result in an under reporting of the EGR flow because it loses its sensitivity. In response, the PCM increases the EGR flow, which leaves the valve open too long and creates a lean condition.

Replace the DPFE and make sure the EGR valve is not clogged with carbon. If there is an EGR problem, you will see a P0401 code.