Description of the P0408 fault code.
OBD2 Error Codes

P0408 Exhaust Gas Recirculation Sensor "B" Input High

26.05.2022 /

P0408 – OBD-II Trouble Code Technical Description

Trouble code P0408 indicates that the PCM has detected a problem with the EGR system. When this error appears on the vehicle's dashboard, the Check Engine indicator will light up, however, it should be noted that in some cars this indicator may not light up immediately, but only after the error has been detected multiple times.

What does the fault code mean P0408?

Trouble code P0408 indicates a problem with the exhaust gas recirculation (EGR) system. This code occurs when the engine control module (PCM) detects a high input signal from the EGR "B" sensor. When this error appears on the vehicle's dashboard, the Check Engine indicator will light up, however, it should be noted that in some cars this indicator may not light up immediately, but only after the error has been detected multiple times.

Fault code P0408.

Possible reasons

Some possible reasons for the P0408 trouble code:

  • Clogged or blocked EGR valve.
  • Malfunction of the manifold air pressure sensor.
  • Problems with the electrical circuit connecting the EGR valve to the PCM.
  • Incorrect installation or malfunction of the EGR valve.
  • Problems with the EGR system itself, such as leaks or damage.

What are the symptoms of a fault code? P0408?

Symptoms for DTC P0408 may include the following:

  • The Check Engine light on the dashboard comes on.
  • Loss of engine power or uneven engine operation.
  • Increased fuel consumption.
  • Increased emissions of nitrogen oxides (NOx) from the exhaust system.
  • It is possible that the vehicle will not pass an emissions test if required by local regulations.

How to diagnose a fault code P0408?

To diagnose DTC P0408, the following steps are recommended:

  1. Check the Check Engine Light: If the Check Engine Light illuminates on your dashboard, connect the vehicle to a diagnostic scan tool to obtain error codes and more information about the problem.
  2. Check connections and wires: Check the condition of connections and wires associated with the Exhaust Gas Recirculation (EGR) system for corrosion, damage, or breaks.
  3. Check the EGR valve: Check the EGR valve for possible defects or blockages. Clean or replace valve if necessary.
  4. Check the sensors: Check sensors related to the EGR system, such as the EGR valve position sensor and manifold pressure sensor, for proper operation.
  5. Check manifold pressure: Use a pressure gauge to check the manifold pressure while the engine is running. Verify that manifold pressures are as expected based on operating conditions.
  6. Check the cooling system: Check the engine cooling system for problems that could lead to elevated manifold temperatures and therefore a P0408 code.
  7. Check Vacuum Lines: Check the vacuum lines connected to the EGR valve for leaks or damage.
  8. Check PCM software: If necessary, update your PCM software to the latest version, as sometimes updates can correct known problems with the EGR system.

After completing these steps, it is recommended to connect the vehicle to the diagnostic scanner again and clear the error codes. If the problem persists and the P0408 code recurs, a deeper investigation or consultation with a professional may be needed.

Diagnostic errors

When diagnosing DTC P0408, the following errors may occur:

  • Incorrect interpretation of error code: Sometimes mechanics may misinterpret the P0408 code and start replacing components that may be fine. This may result in unnecessary repair costs.
  • Insufficient diagnosis: A malfunction in the Exhaust Gas Recirculation (EGR) system can be caused by several factors, and improper diagnosis can lead to the root of the problem not being properly identified.
  • Skipping diagnostics for other components: Sometimes mechanics may focus only on the EGR valve and not check other components such as sensors, wires or manifold pressure, which can lead to an incomplete diagnosis.
  • Malfunction of the scanner or diagnostic equipment: Sometimes errors can occur due to faulty diagnostic equipment or scanner, which may misinterpret error codes or provide inaccurate information about the system status.
  • Fault in other systems: Sometimes manifold pressure or sensor problems can cause P0408 to appear even if the EGR valve is working normally. This may be missed during diagnosis.

To avoid these errors, it is important to perform a comprehensive diagnosis that includes checking all components associated with the EGR system, as well as using reliable and up-to-date diagnostic equipment. If necessary, it is better to contact a professional auto mechanic to accurately diagnose and fix the problem.

How serious is the fault code? P0408?

Trouble code P0408 indicates problems with the exhaust gas recirculation (EGR) system. Although this is not a critical failure, it can lead to a number of problems, including increased nitrogen oxide emissions, decreased vehicle environmental performance, and loss of performance and fuel economy.

Additionally, a P0408 code may cause the vehicle to fail an emissions test, which could render it unroadworthy if the problem is not corrected.

Although the P0408 code is not an extremely serious problem, it still requires careful attention and timely repair to prevent further problems with the vehicle.

What repair will help eliminate the code? P0408?

Troubleshooting DTC P0408 typically involves the following repair steps:

  1. Check the exhaust gas recirculation (EGR) system for blockages or damage.
  2. Clean or replace the EGR valve if clogs are found.
  3. Check the connecting wires and connectors associated with the EGR valve for corrosion or breaks.
  4. Checking the readings of sensors and air pressure sensors in the EGR system.
  5. Check the operation of the electronic engine control module (ECM) for malfunctions or malfunctions.
  6. Clean or replace the filter in the EGR system, if necessary.
  7. Check the vacuum lines associated with the EGR valve for leaks.

After completing these steps, it is recommended that you test for errors and crashes to ensure that the problem is resolved and the P0408 code no longer appears. If the problem persists, more advanced diagnostics or replacement of EGR system components may be required.

How to Fix P0408 Engine Code in 3 Minutes [2 DIY Methods / Only $4.24]

P0408 – Brand-specific information

Trouble code P0408 can be applied to different brands of cars, but the decoding may vary slightly depending on the manufacturer, a list of some car brands with their decodings:

Please refer to the repair manual or service manual for your specific vehicle brand for more detailed information on deciphering the P0408 trouble code.

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