P0400 - Exhaust Gas Recirculation Flow Malfunction
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System
Powertrain (Engine & Gearbox)
Related Fault Codes
Technical Description
P0400 is set when the Engine Control Unit (ECU) detects that the flow of exhaust gases through the Exhaust Gas Recirculation (EGR) valve is outside the calibrated range. The ECU monitors this flow by measuring the change in manifold absolute pressure (MAP) or mass air flow (MAF) when the EGR valve is commanded open. A lack of expected change indicates a restriction or mechanical failure. This failure leads to increased Nitrogen Oxide (NOx) emissions and may trigger a limp-home mode to protect the engine.
Possible Causes
Carbon buildup in the EGR passages or the valve itself restricting flow
Faulty EGR valve (mechanical seizure)
Failed vacuum pump, leaking vacuum lines, or faulty EGR solenoid (on vacuum-controlled systems)
Defective MAP or MAF sensor providing incorrect feedback to the ECU
Symptoms
Recommended Solution
Workflow
Mechanic's Tip
Before replacing expensive components, always double-check wiring and connections. 80% of all sensor faults are actually due to corroded contacts or broken wires.
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