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P0400 OBD-II Trouble Code: Exhaust Gas Recirculation Flow Malfunction

Check Engine Light

P0400 code definition

Exhaust Gas Recirculation Flow Malfunction

What the P0400 code means

P0400 is an OBD-II generic code that the engine control module (ECM) detected the engine exhaust gas recirculation (EGR) valve is not flowing recirculated exhaust gases when it is commanded to open the flow of gases into the intake manifold.

What causes the P0400 code?

  • The EGR valve is not opening due to no vacuum opening the valve from the EGR control solenoid or the EGR not holding vacuum.

  • The EGR temperature sensor is not registering a temperature change when the EGR valve is opened.

  • The EGR passages are blocked and will not allow flow of exhaust gases into the intake manifold.

What are the symptoms of the P0400 code?

  • The Check Engine Light will be illuminated and the code is set in ECM memory.
  • The engine could have an engine ignition ping or knock on acceleration.
  • The engine will fail emissions from excessive NOx in the exhaust tests.

How does a mechanic diagnose the P0400 code?

  • Scans codes and documents the freeze frame data in order to verify the problem.

  • Clears the engine and ETC codes and road tests the car to verify whether the code comes back.

  • Visually checks the vacuum hoses, wiring, and connections to the EGR valve and control solenoid along with the EGR temperature sensor.

  • Disconnects and checks the EGR valve is getting vacuum to the valve when the control solenoid opens on light to medium acceleration.

  • Checks for EGR temperature sensor change and engine to bog down when the EGR is opened.

  • Removes EGR valve and temperature sensor to check for excessive carbon buildup or blockage of EGR tubes.

Common mistakes when diagnosing the P0400 code

  • Replacing the EGR valve before checking for carbon build up on the EGR temperature sensor.

  • Replacing the EGR valve without checking the EGR pressure transducer for proper function.

How serious is the P0400 code?

  • A failed EGR valve can cause the engine to have excessive ignition that can cause internal engine damage to the piston and valves.

  • The Check Engine Light on will cause the vehicle to fail emission testing from excessive NOx gases.

What repairs can fix the P0400 code?

  • Replacing the EGR valve
  • Replacing a broken vacuum line to the EGR valve
  • Replacing an EGR temperature sensor or cleaning the carbon off of it to repair it
  • Cleaning carbon out of EGR tubes to intake manifold

Additional comments for consideration regarding the P0400 code

Code P0400 is triggered when the EGR temperature sensor does not see a temperature change when the EGR is commanded to open. These sensors tend to get a lot of carbon buildup that causes them to lose sensitivity to the heat from the EGR gases.

Need help with a P0400 code?

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