P0400 Exhaust Gas Recirculation Flow Malfunction

Clogged-up EGR valve
Clogged-up EGR valve

the EGR passages and replacing the EGR valve. For some models, the EGR valve has been updated. Your local dealer will be able to confirm if the EGR valve part number has been updated.

On many 90's and early 00's Nissan vehicles (e.g. Pathfinder, Altima, Maxima, Frontier, Quest) the code P0400 could be caused by a carbon buildup blocking the air flow around EGR temperature sensor. The repair involves removing and cleaning the EGR temperature sensor an the EGR tubes, ports and passages. A service bulletin for the 1998-2002 Frontier and Xterra with a 2.4L 4-cylinder engine recommends cleaning the EGR port in the intake manifold; the EGR port plugs need to be removed

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In the 90's and early 00's Mazda cars and trucks, the code P0400 could be caused by clogged EGR passages, often in the throttle body. The repair also involves cleaning the EGR passages. Similarly in the 90's Chevrolet Tracker or its twin Suzuki Sidekick/Vitara, the clogged EGR passages in the intake plenum and the back side of the throttle plate can cause the code P0400. The repair involves checking the EGR operation with a scan tool and if the idle doesn't change when the EGR valve is commanded open, checking and if needed cleaning the EGR passages.

- A clogged EGR tube was a common case for the code P0400 in some late 90's early 00's Mercedes-Benz cars. To clean the EGR tube, the EGR valve needs to be removed. This thread has some very descriptive photos.

How the EGR system works

EGR system diagram
Basic EGR system diagram

The Exhaust Gas Recirculation (EGR) system redirects a small portion of exhaust gases from the exhaust manifold back into the intake manifold (see the diagram) to lower the combustion temperature.
The engine computer regulates the flow of the EGR gases by opening or closing the EGR valve. The EGR flow is at its maximum during steady highway cruising.

The engine computer periodically tests the operation of the EGR system. If the engine computer detects that the EGR flow is less or greater than expected, it sets the fault code P0400 - Exhaust Gas Recirculation Flow Malfunction.

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