P0400 Code: Meaning, Symptoms, Causes & Fixing

The Exhaust Gas Recirculation (EGR) system is most commonly used in modern vehicle models. The EGR system is used to control the emission of exhaust gases. The exhaust gases are produced due to the combustion of the air-fuel mixture in the engine combustion chamber. The EGR system recirculates these gases into the combustion chamber to burn them again. Your powertrain control module (PCM) regularly monitors the working of the EGR system. When your EGR system doesn’t recirculate exhaust gases according to the requirements, your PCM triggers the DTC P0400. This article mainly explains the P0400 code meaning, symptoms, and causes.

P0400 Code Definition

P0400 code stands for “Exhaust Gas Recirculation Flow Malfunction.”

What Does the P0400 Code Mean?

The P0400 code indicates that your powertrain control module (PCM) detects your exhaust gas recirculation (EGR) valve is not flowing recirculated exhaust gases according to the requirements.

P0400 code

Exhaust gas recirculation is a principle of emissions control. The Exhaust Gas Recirculation (EGR) system is one of the major parts of the modern vehicles. This system helps to control the emission of exhaust gases. The emission of exhaust gases into the environment spoils the atmosphere and produces different diseases.

The EGR system recirculates a specific amount of exhaust gases into the engine intake to minimize the combustion temperature and lower the emission of the exhaust gases.

However, the timing of this recirculation depends upon several factors, most notably engine temperature.

Your EGR system has different parts, such as the EGR sensor and EGR valve. The EGR valve is known as the heart of the EGR system. This valve helps to redirect a specific amount of exhaust gases into the intake manifold. This valve also helps to reduce the combustion chamber temperature and the emission of nitrogen oxide (NOx). If any part of the EGR system goes bad, your vehicle can’t pass an emission test.

In the latest vehicle models, the vehicle PCM controls the EGR valve. However, the EGR valves of the old models are controlled by the engine vacuum.

When your PCM observes that the EGR system is not recirculating sufficient exhaust gases to the engine, it triggers the P0400 code.

Symptoms of Code P0400

  • Check engine light illumination
  • Engine stalling
  • Hard to start the engine
  • Rough idling
  • Poor acceleration or a reduction in engine power
  • Poor fuel economy
  • Engine knocking
  • Emission test failure

Read More: Symptoms and Causes of bad EGR Valve

Causes of P0400 Code

  • A bad EGR valve
  • Faulty EGR pressure sensor
  • A vacuum leak
  • Bad catalytic converter
  • Blocked EGR passages
  • Faulty EGR position sensor
  • Faulty EGR temperature sensor
  • Poor connections or wires connected to the EGR control circuit
  • Damaged or disconnected vacuum lines
  • Faulty EGR solenoid
  • A problem with the PCM (i.e., software needs an update)

How to Diagnose the P0400 Code

  • Scan all the trouble codes using an OBD-II scanner and record the freeze frame data in order to confirm the issue.
  • Clear the engine and ETC codes and perform a test drive to ensure whether the code returns.
  • Inspect the vacuum hoses for damage or blockage.
  • Inspect the EGR system connections and wires for corrosion or damage.
  • Properly inspect the EGR valve.
  • Inspect the EGR-related sensors (i.e., EGR position sensor, EGR temperature sensor, and EGR pressure sensor).
  • Properly inspect the EGR control solenoid.
  • Disconnect and inspect the EGR valve is getting vacuum to the valve when the control solenoid opens on light to medium acceleration.
  • Remove the EGR valve and EGR sensors to properly inspect for unnecessary carbon accumulation or blockage of EGR tubes.
  • Check the powertrain control module (PCM).

Common P0400 Code Diagnostic Mistakes

  • Replacing the unnecessary components.
  • Replacing the EGR valve or EGR sensor before inspecting the wiring and electrical connections.
  • Replacing the EGR valve without checking the vacuum system for leaks.
  • Not clearing the PCM memory after fixing the error code.
  • Changing the EGR valve before inspecting the EGR temperature sensor and EGR pressure sensor for carbon buildup.
  • Installing the new EGR valve without testing it.

How serious is the P0400 Code?

In general, the P0400 code is considered a relatively serious trouble code in nature. This code is triggered due to a fault in your EGR system, such as a faulty EGR valve. A bad EGR valve may cause the engine to have excessive ignition that may badly damage the internal parts of your engine, such as valves and pistons.  

However, you can’t pass an emission test with this code. When your vehicle triggers this code, it means that it emits extreme NOx gases that are very hazardous to the atmosphere. Therefore, when your vehicle triggers the P0400 code, you should immediately inspect your EGR system and address the root cause of the problem.

What repairs can fix the P0400 Code?

  • Replacing the faulty EGR valve
  • Cleaning carbon out of EGR tubes to intake manifold
  • Repairing the damaged or leaked vacuum lines to the EGR valve
  • Replacing the faulty EGR temperature sensor
  • Replacing the faulty EGR pressure sensor
  • Cleaning the clogged EGR valve
  • Cleaning the carbon buildup in the EGR temperature sensor

P0400 Code Repair Cost

To fix or repair the P0400 trouble code, you need one or more of the following repairs:

PartsCost
EGR valve$220 to $370
EGR pressure sensor replacement$90 to $130
EGR valve temperature sensor replacement$210 to $240

FAQ Section

What are the signs of a failing EGR valve?

  • Illuminated check engine light
  • Poor engine performance
  • Poor fuel economy
  • Failed emission test
  • Engine knocking
  • Smell of unburnt fuel

Can I drive with the P0400 code?

It depends on the cause of the P0400 code. If this code is triggered due to a minor issue, such as a bad EGR position sensor, while the EGR valve is working properly, there will be no driving issues. But if your EGR valve is bad, you shouldn’t drive your vehicle. Driving with a faulty EGR valve increases the temperature of the combustion chamber, which may cause damage to the engine’s internal parts.

Does removing EGR increase power?

Yes, removing the EGR valve improves your vehicle’s performance and engine power, and it will be a lot more powerful.

What are the causes of P0400?

  • Damaged EGR valve
  • Faulty EGR pressure sensor
  • A vacuum leak
  • Bad catalytic converter
  • Blocked EGR passages
  • Faulty EGR position sensor
  • Faulty EGR temperature sensor
  • Poor connections or wires connected to the EGR control circuit
  • Damaged or disconnected vacuum lines
  • Faulty EGR solenoid
  • A problem with the PCM
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