P0401 — Exhaust gas recirculation flow insufficient

P0401 indicates EGR flow below the monitor's expectation. Depending on system design, the controller may infer flow from air mass, pressure, temperature or other feedback. A valve moving to its commanded position does not prove that enough exhaust gas is passing through the circuit. Assess actuation, sealing and flow restrictions separately before cleaning or replacing the EGR assembly.

Affected system: Exhaust gas recirculation

Symptoms

• Engine warning, sometimes without a noticeable driving change.
• Hesitation or reduced power during operating modes that use EGR.
• Uneven running; other symptoms depend on the actual fault and engine.

Possible causes

• Deposits restricting EGR passages, valve or cooler where fitted.
• Valve or actuator failing to open sufficiently.
• Vacuum leakage or electrical-control fault.
• MAF, pressure/temperature feedback, sensing pipes or wiring misreporting flow.

What to check

• Inspect accessible hoses and connectors before replacing the valve; save codes and freeze-frame data.
• Identify the engine's EGR layout and measurement method; look for accompanying MAF, pressure or DPF faults.
• Compare EGR command, reported position and airflow/pressure response during an approved test.
• For vacuum-operated EGR check supply and sealing; for electrical EGR check supply, ground and command.
• When actuation is correct, investigate restrictions and feedback. Passing an electrical test does not exclude a blocked gas path.
• After repair perform adaptation only if specified and verify flow/monitor operation. Do not apply an engine-specific bulletin to unrelated engines.

Step-by-step repair guide

• EGR valve cleaning: symptoms, causes and step-by-step procedure

Also written as: P0401-00, P040100, PO401 (some scan tools add the fault-type byte, e.g. -00).