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

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Cost of diagnosing the P0400 code

P0400 is a diagnostic trouble code (DTC) for "Exhaust Gas Recirculation Flow Malfunction". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic for $114.99 . Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.0 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.

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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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Related questions

I have codes P0151, P0171, and P0174. I have changed out my O2 sensor, but now I have all these codes
Hey there. The codes you have downloaded simply relate to malfunctions in the "engine management system." It appears codes were set for oxygen sensor (https://www.yourmechanic.com/services/oxygen-sensor-replacement) voltage out-of-range, possible EGR (exhaust gas recirculation) problems such as a blockage, and a generic...
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Diagnostic Trouble Code P0400 (https://www.yourmechanic.com/article/p0400-obd-ii-trouble-code-exhaust-gas-recirculation-flow-malfunction-by-jay-safford) can set if the EGR valve is faulty but other causes for code are a break in the vacuum line to the EGR valve, a faulty EGR temperature sensor, or carbon deposits and blockages in...
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Code P0400 (https://www.yourmechanic.com/article/p0400-obd-ii-trouble-code-exhaust-gas-recirculation-flow-malfunction-by-jay-safford) does not necessarily mean that there is something wrong with the EGR valve itself. The only way to condemn the EGR valve is to directly test it. In particular Code P0400 includes a circumstance where the EGR...

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