P0400 Exhaust Gas Recirculation Flow Malfunction
The Exhaust Gas Recirculation (EGR) system redirects a small part of the exhaust gases from the exhaust manifold through the tubes or passages back into the intake, with the EGR valveFord EGR valve controlling the recirculation flow. The EGR recirculation flow is commanded and monitored by the engine computer (ECM); the ECM can gradually open or close the EGR valve depending on driving conditions. The EGR flow is commanded to its highest level during steady highway cruising. The engine computer (ECM) periodically tests the operation of the EGR system. During a test if the ECM detects that the EGR flow is less or greater than expected, the fault is detected and the code P0400- Exhaust Gas Recirculation Flow Malfunction is stored in the ECM memory. Depending on the problem, a vehicle may also exhibit some driveability concerns, such as, surging, pinging, poor fuel economy, rough idle, etc.
What could cause the code P0400
- clogged or restricted EGR passages
- sticking 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 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
On many 90's -early 00's Nissan vehicles (e.g. Pathfinder, Altima, Maxima, Frontier, Quest) the code P0400 can 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.
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.