P0400 Exhaust Gas Recirculation Flow Malfunction

EGR system diagram
Basic EGR system diagram

The Exhaust Gas Recirculation (EGR) system redirects a small part of the exhaust gases from the exhaust manifold 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 and turns on the "Check Engine" light on the dash. Typically the code P0400 is set when the EGR flow is less than expected. This is usually caused by carbon deposits (soot) from the exhaust clogging up the EGR passages and tubes. This problem is especially common in high-mileage engines or when the engine consumes oil.

• Symptoms
• Is it safe to drive with the code P0400?
• Causes
• Common problems
• What needs to be checked

Symptoms

There might be some driveability concerns, such as, surging, poor fuel economy, rough idle, but in some cases there could no noticeable symptoms at all. In some vehicles, if the EGR system doesn't work, there might be some engine pinging (detonation) noticeable on acceleration or under load.

Is it safe to drive with the code P0400?

It really depends on the cause. If the code P0400 is caused by some minor issue like a faulty EGR temperature sensor or cracked MAP sensor vacuum hose and the EGR system works properly, there should be no problem driving. If the EGR system doesn't work properly, the combustion temperature will be higher, which in a long run could result in burnt valves, blown head gasket, etc. Remember, the purpose of the EGR system is to lowers the combustion temperature.

Causes

- clogged or restricted EGR passages
- sticking or clogged EGR valve
- problem with EGR valve position sensor
- cracked or restricted vacuum line to the manifold absolute pressure (MAP) sensor
- clogged catalytic converter
- carbon deposits (soot) on the EGR temperature sensor (Nissan)
- faulty mass airflow sensor (MAF)
- open or short in the EGR temperature sensor circuit
- mis-routed vacuum lines
- electrical problem with the EGR valve control circuit
- engine computer problems

Common problems

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.
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 Mercedes-Benz cars. To clean the EGR tube, the EGR valve needs to be removed. This thread has some very descriptive photos.

What needs to be checked with the code P0400:

The most often found problem with the EGR system is carbon buildup restricting the EGR flow. Usually, if a carbon buildup is found in one part of the EGR system, the other ports and passages could be clogged too, so the entire EGR system needs to be checked for restrictions and cleaned if needed to fix the problem. Check the vacuum line routing according to the vacuum line diagram. Check the electrical part: the connectors and the wiring to the EGR valves, solenoids and sensors. In Mazda vehicle, the EGR boost sensor and its vacuum hose need to be checked as the sensor could fail or the sensor tube could be restricted or cracked. In a Subaru, the BPT valve needs to be checked along with other EGR components. If a clogged catalytic converter is suspected, the exhaust back pressure needs to be checked. In Nissan vehicles the EGR temperature sensor clogs up often; it needs to be inspected first.

If the problem is fixed, will the "Check Engine" light come off by itself?

If you have found and repaired the problem causing the code P0400, the "Check Engine" light will turn off by itself after driving for a day or two. The "Check Engine" light can also be reset by disconnecting and reconnecting the battery. Of course, the code P0400 will come back if the problem is still present.