P0401 Ford F150 – Diagnosing and Fixing the Problem

P0401 is one of the most common diagnostic EGR codes found in Ford trucks like the F150. Aside from Honda, Acura, Toyota Camry, or Corolla, this code is prevalent in Ford trucks.

P0401 code highlights

• Level of urgency:Medium
• Possible culprits:EGR valve problems
• Price for repair:$200 - $800
• If neglected:Bad efficiency, harsh work, loss of power
• DIY repairs:Impossible
• Can you drive?Carefully

What Does P0401 Mean for Ford F150?

P0401 in F150 means EGR Insufficient flow. This diagnosis only means that the EGR's recirculated gases have been restricted and moving too slowly.

What Does Insufficient Flow of Recycled Gases Mean For the EGR?

F150 is equipped with an Exhaust Gas Recirculation System. It is responsible for taking in exhaust gases from the intake, regulating pressure and temperature, and reducing the NOx emissions from the vehicle. For the EGR to perform its job, it has to maintain a specific exhaust gas flow rate to direct back on the intake.

To do this, the EGR of an F150 also employs a Pressure Feedback System to measure the flow rate of exhaust gases. It uses Differential Pressure Feedback Electronic (DPFE) Sensors, which tell the Power Train Module how much exhaust gas flows on the EGR.

For F150, and most Ford trucks, Most P0401 codes arise from defective DPFE sensors. When faulty, it cannot retrieve information or send signals to the PCM correctly.

A P0401 code means not simply a restriction of exhaust gas flow because of faulty DPFE sensors but issues from different parts of the EGR itself.

The EGR system uses a valve that opens and closes and regulates the flow of exhaust gases in the system. It connects the exhaust and intake manifold. It is often inactive when the engine is idling or cold and when hard accelerating. Moreover, this valve is most active when the engine is moving.

Another critical part of the engine system is the EGR vacuum solenoid, an electronic control that directs part of the exhaust gases from the exhaust to the intake manifold. It supplies the proper amount of vacuum to the EGR. Any restriction or resistance from this control will trigger the P0401 code.

What Causes P0401 Code in Ford F150?

Let's talk more about these causes in detail.

• Faulty DPFE sensor

The most common culprit of the P0401 code for the Ford F150 is the faulty DPFE sensor. It is so common that most technicians would test this sensor and the hoses attached to it first.

This sensor measures the pressure of incoming and outgoing airflow, sending it to the PCM. The computer decides how much exhaust gases should get in or out of the intake and into the engine.

Over time, the diaphragm becomes less flexible and functional. Another culprit in faulty DPFE sensors is moisture and corrosion inside their box. It eventually reads the wrong flow rates and sends them to the PCM. Incorrect readings result in improper commands by the computer, affecting the supply of exhaust gases. Any event that the computer cannot detect or measure would trigger the P0401 code.

• Damaged sensor tubes

DPFE sensors need properly sealed hoses without cracks or any restrictions. The trouble during hose repairs is it can sometimes reverse, keeping the P0401 code after repair. In these cases, upgrading the parts would solve the problem.

• Blocked EGR valve

The EGR cools the temperature before sending the exhaust gas into the air. This valve deals with exhaust gases to recirculate them back to the engine, making it prone to carbon build-up. As carbon build-up increases, it restricts the exhaust gases recirculating to the intake. Blocked carbon deposits not only restrict the EGR flow but also affects the function of the entire system.

When the system has not achieved the required flow rate by your F150, it will trigger the Malfunction Indicator Lamp (MIL) in Ford trucks, otherwise known as the Check Engine Light. Along with it is the P0401 for your F150.

• Clogged Throttle Body Ports

Aside from the blockages in the EGR valve, look into other passages and the ports in the throttle body. Two ports in the throttle body usually clog up. Clear them up because they are the common blockages in F150 EGR systems.

• Failing EGR Vacuum Modulator Valve

The EGR vacuum modulator valve turns the vacuum on and off to make the exhaust gases flow according to the temperature. When the vacuum modulator fails, your EGR will not work properly. It can result in poor acceleration, increased emissions, and engine damage due to overheating.

The EGR vacuum modulator fails due to worn-out or damaged components, such as the solenoid or diaphragm. Next, it can become defective because of carbon build-up, blockages on passages, and faulty vacuum passages.

• Bad EGR Valve

EGR Valve is one of the most critical parts of an EGR system. A faulty EGR valve can cause different performance issues: reduced fuel economy, stalling engines, and increased emissions.

The EGR valve has to deal with passing exhaust gases that can catch carbon deposits, fuel residues, dirt, additives, and more. Over time, the EGR valve accumulates dirt and debris harmful to the entire system.

The presence of carbon deposits in the EGR valve can affect its performance to make or shut the way for exhaust gases. Aside from clogs, a faulty solenoid, a vacuum leak, or poor wiring connections can damage the EGR valve.

• Malfunctioning EGR Solenoid Valve

An activated EGR solenoid will open up a passage that lets exhaust gases into the intake manifold. It controls the right amount of exhaust gases in the intake. It can damage due to faulty wiring connections, worn-out seals, and malfunctioning PCM. When this component malfunctions, it will result in an incorrect exhaust gas supply, triggering a P0401 code.

• Open or Shorted EGR Solenoid Valve

An open or shorted EGR solenoid valve is caused by a defective wiring harness or a faulty solenoid. The corroded wiring harness will keep the solenoid valve open when it needs to close. On the other hand, the EGR solenoid remains closed when it should open if the valve has wiring problems or a defective solenoid. Should this valve remain closed when it needs to open, there will be no gas recirculation. This event will affect the performance of the engine.

• Faulty EGR Temperature Sensor

The EGR temperature sensor monitors the temperature of recirculated exhaust gases. These sensors could become defective because of corrosion, poor connections, a faulty sensor, mechanical failure, dirt & grime, and poor installation.

Since a faulty EGR temp sensor does not work correctly, regulation of the rate of exhaust gas recirculated will be affected, resulting in reduced or increased flow rate. Worst comes to worst, an unreplaced defective EGR temperature sensor may cause the engine to go into limp mode.

• Intake Manifold Carbon Build Up

Carbon build-up in the intake manifold will restrict the recirculated gas to the EGR valve, affecting the way it opens and closes. This problem will affect successful gas recirculation back to the F150 engine and result in more oxygen in the air-fuel mixture, resulting in poor engine performance.

In these cases, your best way to prevent carbon build-up is to bring them to your service shop for inspection as scheduled. Use the appropriate carbon foam cleaner for the intake manifold, ports, and throttle bodies.

Symptoms of P0401 in F150

Apart from the code P0401, a lot of signs can help you diagnose the problem in the EGR system.

• Malfunction Indicator Lamp (MIL) or check engine lights on
• Engine hesitation
• Detonation (Ping or knocks during acceleration)
• Failed NOx emission test

How to Diagnose P0401 in your Ford F150?

Technicians would start with an OBD-II scanner to obtain the freeze frame data for faster diagnosis. He would reset the computer to see if the codes would appear again.

The next step is inspection. Any technician would test the DPFE sensors first because it is a common occurrence for P0401 in F150. But you can do this yourself with a voltmeter or a multimeter. Depending on its range, a Ford DPFE should measure between 0.4 and 0.9 Volts. Any measure out of this range means you should replace the DPFE sensors.

Next, proceed with inspecting the DPFE tubes for cracks. If it is good, check the throttle body ports for blockages. Remove the throttle body to get a good view. Since you already removed it, clean it.

After cleaning the throttle body ports, check for the EGR solenoid. It might be stuck open. You only need to remove the hoses from the port and blow them with spare hoses. Should you hear air escaping, replace it.

Don't forget to check if your temperature sensor is working too. You can test voltage and resistance by using a digital multimeter. The temperature sensor has a specified range by Ford.

You can replace the sensor should any resistance or voltage is too low in its range. If the resistance and voltage are too high, clean it and test it again.

Finally, check the EGR valve. Use a vacuum gauge for this. Set it to zero and pump the engine. You must observe it rising gradually if it is functioning. If the gauge doesn't move, you need to replace it. Also, check if the reading is within its range. When it measures too low, it must be blocked. When they are too high, the valve must have been stuck open.

How Serious is a P0401 Code in F150?

Luckily, replacing components on a P0401 is not costly. However, a defective EGR valve may damage your piston and valves with too much ignition pre-ignition. Also, it would cause failed emission tests.

How to Prevent the P0401 Code in F150?

You can perform maintenance measures to prevent P0401 from happening again. Change the DPFE sensor diaphragm at 140,000 to 150,000 miles.

Clean the EGR ports every 50,000 miles or as needed.

Also, make it a habit to check your DPFE hoses after changing the oil.