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P0408 OBD-II Trouble Code: Exhaust Gas Recirculation Sensor B Circuit High

check engine light

P0408 code definition

The P0408 code is stored when the PCM detects an issue with the EGR (exhaust gas recirculation) system. After one or more (depending on the vehicle manufacturer’s specifications) instances of this, the code will be stored, and the Check Engine Light will illuminate.

What the P0408 code means

When the EGR system is working properly, it will recirculate inert exhaust emissions back into the engine cylinder. These gasses burn at a lower temperature, thereby lowering the temperature of combustion for the engine fuel, which reduces the emission of nitrogen oxides. When the EGR system is activated, the PCM should detect a certain degree of manifold air pressure change.

If it detects a change that is outside the acceptable range, it will record the P0408 code. Some manufacturers’ vehicles will record the code after one instance, while others will record it after multiple instances.

What causes the P0408 code?

A few things could be occurring to cause a P0408 code to be stored. These include:

  • A damaged or defective EGR temperature sensor
  • A fault in the EGR temperature sensor’s circuit
  • A shorted or open connection in the EGR volume control solenoid valve circuit
  • Excessive carbon buildup on the EGR’s valve
  • A damaged or defective EGR volume control solenoid valve
  • An open ground circuit for the EGR sensor
  • An internal failure in the solenoid or EGR sensor

What are the symptoms of the P0408 code?

When the P0408 code is stored, the Check Engine Light will always illuminate. Other than this symptom, there may be no discernible change in the performance of the vehicle. However, some other symptoms that the driver may notice include:

  • Engine surging during operation
  • Intermittent engine stalls

There will likely be an increase in nitrogen oxide emissions, due to the increased combustion temperature, but this will usually be undetectable to the driver or owner of the vehicle until an emissions test is performed.

How does a mechanic diagnose the P0408 code?

The first step that any mechanic will take to diagnose a P0408 code is to use an OBD-II scanner to identify it and any other codes stored by the PCM. Then, if the vehicle stalls after starting and/or will not run consistently, they will generally unplug the EGR valve before clearing the code and then restarting the vehicle.

If the code appears again, they’ll then remove the EGR valve and clean out any built-up debris before reinstalling it and starting the vehicle again. If the problem still isn’t solved, they will block the EGR’s port and try to crank the engine. If this seems to solve the problem, then the cause of the code is most likely a faulty EGR valve, which is stuck in the open position. Replacing it should resolve the code and allow the engine to start consistently.

If the EGR valve is vacuum controlled, and unplugging it works to allow the vehicle to allow the vehicle to start and run, the problem is likely with the EGR solenoid. If this is not the case, the mechanic will continue taking steps to diagnose the problem, including visual inspections of all wiring, circuits, and connections. After each test and repair is done, the mechanic will clear the code and retest it to ensure that it does not appear again.

Common mistakes when diagnosing the P0408 code

The most commonly reported misdiagnosis of a P0408 code is immediately assuming that the EGR valve is the problem. Replacing this without inspecting any other potential problems will result in spending unnecessary time and money without actually addressing the problem causing the P0408 code to be recorded.

What repairs can fix the P0408 code?

Some common fixes for a P0408 code include:

Need help with a P0408 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
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