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  • Diagnostic trouble code (DTC) P0400 code stands for “Exhaust Gas Recirculation (EGR) Flow Malfunction.”
  • The code is logged once the flow of exhaust gas recirculation is lower than what the powertrain control module (PCM) expects.
  • Aside from the check engine light, you’ll usually experience engine hesitation, rough idling, reduced power, poor acceleration, engine knocking (or detonation), and poor fuel economy.

OBD-II codes are standardized codes that point toward issues that could increase tailpipe emissions in vehicles, making it easier to diagnose and troubleshoot problems. If your OBD-II scanner registered a code P0400, that indicates a potential problem with your vehicle’s exhaust gas recirculation (EGR) system. Learn how to address it by learning about its meaning, causes, symptoms, and more.

What Does the P0400 Code Mean?

Diagnostic trouble code (DTC) P0400 code stands for “Exhaust Gas Recirculation (EGR) Flow Malfunction.” It is triggered when the flow of exhaust gas recirculation is lower than what the powertrain control module (PCM) expects.

The heart of the EGR system is the EGR valve, which redirects a portion of the exhaust gas into the engine intake manifold to be integrated into the air/fuel mixture. This reduces cylinder temperature and the amount of nitrogen oxide (NOx). If it malfunctions, your car may fail an emission test.

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Closeup of exhaust gas recirculation in engine compartment
Diagnostic trouble code P0400 is triggered when the flow of exhaust gas recirculation is lower than what the powertrain control module (PCM) expects.

In modern vehicles, the EGR valve is controlled by the PCM. Meanwhile, on older vehicles, the valve is usually controlled directly by the engine vacuum.

If the PCM detects that there is insufficient exhaust gas being sent into the engine, code P0400 will be triggered. This means that the system is not recirculating exhaust gas under conditions that require EGR.

An in-depth knowledge of the EGR system can help you come up with a proper DIY fix for P0400. Otherwise, if you’re not planning to troubleshoot the code yourself, you can continue reading about the possible causes of this code.

What are the Possible Causes of the P0400 Code?

The causes of the P0400 code may differ depending on the make and model of your car. Here are the most common causes of an EGR flow malfunction or code P0400:

  • Faulty EGR valve
  • Clogged EGR passage
  • Disconnected or damaged vacuum lines
  • Faulty EGR-related sensor (e.g., EGR temperature sensor, EGR pressure sensor, etc.)
  • Issues with the EGR control circuit, such as damaged wires or poor connections
  • An issue with the PCM, such as software in need of an update
egr pressure sensor
Issues with the EGR Pressure Sensor can also trigger the P0400 code.

What are the Common Symptoms of the P0400 Code?

Now that you know what causes the P0400 code, it’s time to learn how to identify it. You may encounter issues that concern the driveability of your car if code P0400 is triggered. Here are the common symptoms of this DTC:

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In some cases, the vehicle will exhibit no symptoms other than an illuminated check engine light or malfunction indicator lamp (MIL).

How to Diagnose the P0400 Code

Many issues can trigger the P0400 code, which can make it difficult to identify its exact cause. Leave this task to an expert if you are not well-versed in automotive repairs. Otherwise, you may diagnose the problem yourself. Watch these helpful videos to learn more about the P0400 code and get an idea of what its diagnostic process might involve:

How to Fix the P0400 Code

To fix the P0400 code, you must identify and resolve its exact cause. You also need to make sure that the solution you implement is applicable to your vehicle’s make and model. If you are uncertain of your automotive know-how and DIY skills, letting your trusted mechanic handle the job may be your best option. Otherwise, begin the repair process.

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Pinpoint what triggered the P0400 code. With the help of online auto repair resources and guides, determine the most effective way to resolve the issue. Get an ALLDATA single-vehicle subscription for comprehensive factory repair information that will definitely prove helpful in fixing not only the P0400 code but also any other future issue you may encounter.  

Finally, keep in mind that the repair instructions for the P0400 code may vary based on the vehicle’s make and model.

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About The Authors
Written By Automotive and Tech Writers

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.

Reviewed By 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.

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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