P0408 Code - Exhaust Gas Recirculation Sensor 'B' Circuit High

What Does P0408 Mean?

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.

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What are the common causes of P0408 ?

Depending on year, make, and model, DTC P0408 may have number of causes. Here are some of the most common.

• Sensor Fault – Because it is constantly subjected to heat from the exhaust, internal electrical faults are a common occurrence. Replace the sensor, clear the code, keep driving.
• Circuit Problem – Certain repair procedures may require disconnecting the EGR position sensor or interfering with its harness. Poor connections, poor routing, chafing, open and short circuits are par for the course. Repair as needed.
• Dodge / Chrysler / Jeep / RAM / MOPAR – A number of models in this lineup have released TSBs (technical service bulletins) addressing DTC P0405 / P0406 / P0407 / P0408. Reprogramming the ECM fixes the problem. Check with your dealership to see if your vehicle is covered by any of these TSBs.

How do you troubleshoot P0408 ?

If you find a related TSB, the information will often help in diagnosing the code in question as TSBs are crested from many thousands of repairs. With the valve disconnected, the red voltmeter lead should be attached to the IMRC valve power wire and the black voltmeter lead connected to ground. Oxygen sensors signal the engine management computer the amount of oxygen present in the exhaust, which is used in determining and controlling the fuel ratio.



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