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  • Diagnostic trouble code (DTC) P0401 stands for “Exhaust Gas Recirculation (EGR) Flow Insufficient Detected.”
  • The code is logged once the PCM detects that there’s insufficient exhaust gas recirculation (EGR) flow in the system.
  • For diagnostic aids, run a KOER self-test and look for DTC P1408 as a sign of a hard fault. Look for contamination, restrictions, leaks, and intermittents if P1408 is not present.

The error code P0401 is one of the 20 most common OBD-II codes faced by a lot of car owners, although it can be more prevalent in certain makes, such as Toyota and Honda vehicles.

Learn more about what a P0401 code means and how you can diagnose and fix the underlying problem that’s causing it.

What Does the P0401 Code Mean?

Diagnostic trouble code (DTC) P0401 stands for “Exhaust Gas Recirculation (EGR) Flow Insufficient Detected.” This code means that the computer detects insufficient exhaust gas recirculation (EGR) flow in the system.

If by looking at the feedback loop (whatever it may be on the system in question), if the ECM/PCM determines there is insufficient EGR flow, it will set the OBD-II code P0401. Usually on an OBD2 system that sets the P0401, the ECM/PCM must see the failure on two consecutive trips before it will permanently set the code and trigger the Check Engine Light.

The EGR system is important because it reduces emissions by allowing burnt exhaust gas to flow back into the engine. This helps lower your engine’s combustion temperature for a reduction in oxides of nitrogen (NOx) emissions.

egr valve of a car
Code P0401 means that the computer detects insufficient exhaust gas recirculation (EGR) flow in the system.

To learn more about how the EGR system works and how it developed as an emission control technology, check out our in-depth look.

P0401 on Some Ford Vehicles

Code P0401 may appear on some Ford vehicles. On a 1999 Ford F-150 5.4L V8, for example, this DTC can be triggered by the following issues:

  • Vacuum supply
  • EGR valve stuck closed
  • EGR valve leaks vacuum
  • EGR flow path restricted
  • EGRVR circuit shorted to PWR
  • VREF open to DPF EGR sensor
  • DPF EGR sensor downstream hose off or plugged
  • EGRVR circuit open to PCM
  • VPWR open to EGRVR solenoid
  • DPF EGR sensor hoses both off
  • DPF EGR sensor hoses reversed
  • Damaged EGR orifice tube
  • Damaged EGRVR solenoid
  • Damaged PCM
See also  P0402: Exhaust Gas Recirculation “A” Flow Excessive Detected

The EGR system is monitored during steady state driving conditions while the EGR is commanded on. The test fails when the signal from the DPF EGR sensor indicates that EGR flow is less than the desired minimum.

For diagnostic aides, it is advised to perform a KOER self-test and look for DTC P1408 as an indication of a hard fault. If P1408 is not present, look for contamination, restrictions, leaks, and intermittents.

What are the Possible Causes of the P0401 Code?

Code P0401 is experienced by all car types that have on-board diagnostics. Trouble codes on cars with pre-OBD-II systems may vary depending on the make and model year.

Here are the common causes of P0401:

  • Clogged EGR passages
  • Faulty EGR valve
  • A failed sensor (e.g., differential pressure sensor, manifold absolute pressure sensor, etc.)
  • Inadequate vacuum supply to the EGR valve
  • Circuit problems, such as damaged wiring and loose connections
  • Issues with the PCM, such as software in need of an update
Circuit problems in your car
Circuit problems, such as damaged wiring and loose connections are one of the causes of P0401 code.

What are the Common Symptoms Of The P0401 Code?

You may begin experiencing these symptoms if your computer’s giving you a P0401 code:

  • Illuminated malfunction indicator lamp (Check Engine Light)
  • Failed emissions test
  • Engine knocking or pinging during acceleration

How to Diagnose the P0401 Code

Obviously, there is more than one potential reason that a code P0401 might be set, and diagnosing the root cause can be a little tricky for the untrained eye. If you’re not confident with the technical knowledge you have, seek a certified mechanic’s help instead of fixing the problem yourself.

But if you would like to diagnose the problem yourself, check out the videos below to get an idea of how to troubleshoot a P0401 code:

How to Fix the P0401 Code

There’s more than one way of fixing any issues with your car, especially if you’re dealing with OBD-II codes. The exact repair would depend on the root of the problem, and there are multiple reasons why code P0401 might be stored.

Unfortunately, there isn’t a “magic bullet” fix for the issue. You’ll need to diagnose the code accurately to find out what’s triggering the P0401 in your vehicle before you can figure out your plan of attack.

Also, keep in mind that all vehicles are different. When troubleshooting and repairing diagnostic trouble codes, you should consult the factory repair information for your application.

Repair manuals, such as those from Chilton, are useful, but an ALLDATA subscription is even better. ALLDATA has single-vehicle subscriptions for DIYers that provide detailed factory repair information.

P0401 Repair Costs

Code P0401 warns of problems with your EGR system, from a faulty valve to a dirty temperature sensor. The severity of the problem varies depending on the extent of the components affected.

See also  P0406 Code: Exhaust Gas Recirculation Sensor A Circuit High

Total repair costs range from $150 to $750, inclusive of labor and parts. It is important that you pay close attention to your vehicle’s warnings. If you get a P0401 trouble code, immediately schedule an appointment with your trusted mechanic.

An In-Depth Look at the EGR System

The ambient air drawn into the engine is 78% nitrogen, 21% oxygen, and a lot of trace elements. That’s important information for those who want to understand why Exhaust Gas Recirculation came about as an emission control in the early 1970s.

The ambient air drawn into the engine is 78% nitrogen, 21% oxygen, and a lot of trace elements. That’s important information for those who want to understand why Exhaust Gas Recirculation came about as an emission control in the early 1970s.

Richard McCuistian, ASE Certified Master Automobile Technician

During an ignition event, combustion chamber temps when the engine is under load can surpass 2,500° F (1371° C), and at those temperatures, the molecules of inert nitrogen superheated by the uniting of O2 with HC can bond to some of the O2 molecules to form oxides of nitrogen (NOx). An x is used rather than a number because one molecule of N can bond to various different numbers of O molecules; the number isn’t always the same, thus the term OXIDES of nitrogen. NOx is not our friend – so the EGR system was designed to mostly prevent it from forming in the first place. The light-off catalyst takes care of any NOx that manages to form in spite of EGR.

Since exhaust gas leaving the combustion chamber is inert (meaning it won’t burn), about 5% of the exhaust gas on average is recirculated to prevent the combustion chamber temperatures from exceeding the 2500 threshold, but EGR on a gas burner only opens when the engine is off idle and under load. The exhaust gas usually travels through a steel pipe from the exhaust manifold to the valve, which provides the exhaust gas to cool just a bit as it flows.

What is an EGR Valve?

The EGR valve is a device with an internal pintle that is lifted off its seat to allow that very small amount of exhaust gas to enter the intake stream. The earliest EGR valves developed for carbureted engines use a vacuum diaphragm to open the pintle when the throttle is opened, with the vacuum passing through a thermal vacuum switch on its way to the diaphragm. The thermal switch prevents EGR flow when the engine is cold.

The exhaust flow into the intake stream happens very naturally when the EGR valve is open, because the pressure in the exhaust is always greater than the pressure in the intake when the engine is running. Typically the recirculated exhaust will be fed into the intake right behind the throttle plate or through a very small port in each intake runner to make sure every cylinder gets exactly the same amount of EGR flow. Sometimes on the small-port systems (some Nissans and Fords have this), a few of the ports will clog, causing a surge or an engine skip due to uneven EGR delivery between cylinders.

The problem with that early system was that the EGR could be working or not working and nobody would be the wiser. Granted, some engines will ping (labor knock) when EGR is not flowing, but some won’t. Later EGR valves use small electric motors internal to the valve and a three-wire feedback sensor to report EGR pintle position. Early fuel injected Fords had a vacuum solenoid and a three wire linear EGR Valve Position (EVP) sensor that measured diaphragm movement. But monitoring pintle position doesn’t prove EGR gas is flowing into the intake.

See also  P0400 Code: Exhaust Gas Recirculation (EGR) Flow Malfunction

EGR in Pre-OBD2 Vehicles

Pre-OBD2 GM vehicles will watch O2 sensor activity while the ECM is commanding EGR flow, because O2 sensor readings will drift slightly rich when EGR is flowing due to the displacement of oxygen by the inert exhaust gas entering the intake. Pre-OBD2 Ford engineers developed a different feedback system, with a 3 wire pressure sensor that first appeared on Lincoln Town Cars in 1991. That sensor has two silicone hoses connected to the EGR feed pipe with an orifice between the two hoses. When EGR is flowing, the pressure will rise in the hose connected below the orifice and drop in the hose connected above it, thus the Engine Controller (ECM/PCM, etc.) can determine if EGR is flowing and how much.

Pre-OBD2 Asian vehicles are often fitted with a two wire EGR temperature sensor that will report hot exhaust gas when the valve is open – or the lack of hot exhaust gas if there is no flow.

These feedback methods are all about closing the loop. OBD 2 systems are required to monitor EGR flow and store a P0401 code if exhaust flow is insufficient.

Products Mentioned in this Guide

About The Authors
Richard McCuistian, ASE Certified Master Automobile Technician
Reviewed By Richard McCuistian, ASE Certified Master Automobile Technician

Technical Reviewer at

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

CarParts Research Team
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The Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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Dog pit

I keep getting code P0401 I replace hose and egr sensor . Can you let me know what does Repositioned Differential Pressure Feedback EGR (DPFE)? do I need to cut more on the hoes to make sure it doesn’t have extra hose?

Marc Liebeskind

Thank you for this video. Made it easy for me to fix my 2000 Taurus with a P0401 code. Turned out to be a loose connection to the DPFE.

Hi Marc,

So glad you found the information in the article to be helpful!

Dog pit

I keep getting code P0401 I replace EGR SENSOR AND HOSES
Can you let me know what does this means -Repositioned Differential
Pressure Feedback EGR (DPFE)
Sensor Hoses? 03 NAVIGATOR… Thanks

James Sims

From what I am reading it seems that a failing EGR should create some engine performance issues. I have a P0401 code but there are no performance issues.

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