Code Definition

P0400 — Exhaust Gas Recirculation Flow Malfunction

System: Powertrain  |  Type: Generic (OBD-II Standard)  |  Make: Generic

About P0400

P0400 is a generic (obd-ii standard) powertrain diagnostic trouble code. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P0400 is stored, the module has detected a signal or supply voltage lower than the expected operating range.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

Because P0400 is a generic OBD-II code, its base definition is shared across manufacturers. The specific diagnostic steps, reference values, and repair procedures can vary by vehicle, and the TaT Diagnostic Assistant includes verified procedures submitted by working technicians.

Code Information

he ECM/PCM sets this code when it detects insufficient or excessive exhaust gas recirculation (EGR) flow compared to expected values. The EGR system reduces combustion temperatures to lower nitrogen oxide (NOx) emissions. A malfunction can be caused by a clogged EGR passage, faulty EGR valve operation, or a sensor issue.

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