P0401 - Exhaust Gas Recirculation Flow Insufficient Detected

EGR flow is lower than expected.

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Code Overview

EGR flow is lower than expected.

Common Causes

• Carbon buildup restricting EGR passages
• Faulty or sticking EGR valve
• Vacuum control problem on older systems
• EGR flow feedback sensor reading incorrectly

Symptoms

• Check-engine light with little drivability change
• Light pinging or spark knock under load
• Failed emissions test or elevated NOx

Diagnostic Steps

1. Inspect and clean the EGR valve and passages if carbon buildup is visible
2. Command the EGR system and verify the valve actually moves
3. Check vacuum supply or control-solenoid operation on vacuum-operated systems
4. Compare EGR feedback or DPFE readings to commanded flow if equipped

Repair Difficulty

Moderate

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