How to Fix P0400 OBD2 Code?

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If your vehicle is creating a problem of excessive ignition and you are afraid to run it, it might be something serious. For example, a faulty EGR valve can result in excessive ignition, harming the pistons and valves inside the engine. In addition, the presence of the “Check Engine Light” will result in the vehicle failing emission testing due to an excessive amount of NOx emissions. So, running OBD2 diagnostic codes on your malfunctioning EGR will result in popping up the P0400 error code.

If you are looking for a more straightforward guide to fixing the P0400 OBD2 code, we are here to assist you. So, let us get started.

What is P0400 OBD2 Code?

P0400 is an OBD2 generic code that shows that the engine control module (ECM) has detected that the engine exhaust gas recirculation valve does not flow recirculated exhaust gases when the valve is directed to open the gas flow into the intake manifold.

If there is an irregularity in the system, the automobile generates a DTC, and the automobile enters Limp mode to prevent further damage. Limp mode indicates that the Check engine light has been illuminated and that the automobile needs urgent attention to be repaired.

The EPA, MOT, maintenance, and inspection businesses pay close attention to this code, and if there is oil in the intake, the car may be deemed unroadworthy. In addition, the presence of the Check Engine Light will result in the vehicle failing emission testing due to an excessive amount of NOx emissions.

Symptoms of P0400 OBDII Code

If your vehicle has a P0400 error code, it will exhibit the following symptoms:

  • The Check Engine light turns on, and the code is stored in the ECM memory.
  • When the engine is accelerated, it is possible to hear an engine ignition ring or knock.
  • The engine fails the emissions tests because of high NOx emissions in the exhaust.

Causes of P0400 OBDII Code

There are many possible causes of the P0400 error code, and the most common ones are as follows:

  • The EGR valve does not open because the EGR control solenoid does not generate enough vacuum to open the valve, or the EGR does not hold a vacuum.
  • When the EGR valve is opened, the temperature sensor in the EGR system does not report a temperature change.
  • The EGR passageways are clogged, and the exhaust gases will not flow into the inlet manifold as a result.

How can you Diagnose the P0400 Error Code?

Diagnosing the P0400 error code is not that challenging, and you can save your pocket by doing it yourself. So, to do it yourself, follow the below-mentioned guidelines:

  • To validate the problem, you need to scan, code, and document the freeze-frame data.
  • Erase all ETC and engine codes from the car’s computer and drive it to see if the code comes back or not.
  • Evaluate and test the vacuum hoses, connections, and wirings to the EGR valve and control solenoid as well as the EGR temperature sensor.
  • During mild to medium acceleration, only when the control solenoid opens, disconnect the EGR valve and check to see if it holds a vacuum.
  • When the EGR is opened, it looks for changes in the EGR temperature sensor and the engine to bog down.
  • Remove the temperature sensor and EGR valve to inspect the EGR tubes for excessive carbon buildup or obstruction.

How to Fix P0400 OBD2 Code?

You must take a few things into account to correct the P0400 error code. Your thorough attention is required when fixing the P0400 OBD2 code. Some methods for correcting this OBD Code are as follows:

  • Replace the faulty EGR valve with a new one.
  • It is necessary to carefully replace the broken vacuum line leading to the EGR valve.
  • The EGR temperature sensor must be replaced as soon as possible.
  • Ensure that the carbon from the EGR tubes to the intake manifold is removed.

Note: If you experience any difficulties, try to get the assistance of a professional mechanic. 

How to Troubleshoot the P0400 OBD2 Code?

The diagnostic and restoration methods for any EGR code are dependent on the type of EGR valve that has been installed since different forms of EGR valves fail for many reasons. The troubleshooting techniques will also help you a lot to cope with this. So, let us get towards them: 

  • Make a note of all available fault codes and any freeze frame data that is accessible.
  • Inspect all linked vacuum lines for indications of damage or apparent leaks, and replace them as necessary.
  • All associated wiring should be checked for damage; shorted, burned, broken, or rusted wiring and connectors should be avoided at all costs. 
  • Remove the vacuum line from the EGR valve and plug the open end if everything appears to be working.
  • If applied vacuum impacts idling, the vacuum control solenoid is likely to malfunction.
  • If all acquired readings are within requirements, replace the solenoid; otherwise, make modifications to the related connections to verify that all values are within the manufacturer’s specifications. 
  • After the repairs have been completed, clear codes from the computer and check the EGR system to determine whether the code returns.


Can you drive with the P0400 code?

It is dependent on the underlying cause. For example, suppose the code P0400 is induced by a minor issue, such as a malfunctioning EGR temperature sensor, and the EGR system is fully functioning. In that case, there should be no difficulty driving the vehicle. However, if the EGR system is not functioning effectively, the combustion temperature would be elevated, resulting in engine damage over time.

Can a bad EGR valve cause a misfire?

A defective EGR valve might result in a misfire. It is mainly caused by clogged channels in your vehicle’s intake system. It will also harm the power output of your vehicle. Carbon buildup within the EGR valve is exacerbated by oil vapors in the engine, resulting in the valve being jammed.


A code such as P0400 is not something that you can ignore. If left unchecked, it can cause severe damage to the engine of the vehicle. Luckily, there is a proven process for determining the root cause of a code, and the fixes are straightforwardly mentioned in the post. 

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