Fault code P0400 – a step-by-step guide to diagnosing and fixing it

Error code P0400 is displayed and the check engine light is on - what does this mean?

The OBD/EOBD system has detected a fault in the exhaust gas recirculation system (EGR system). This DTC P0400 indicates insufficient or blocked exhaust gas recirculation, which can occur with various manufacturers such as Volkswagen, BMW, Mercedes-Benz and others.

What causes can this error code have?

• Blocked or defective EGR valve
• Blocked EGR lines due to soot deposits
• Defective EGR position sensor
• Damaged vacuum lines to the EGR system
• Faulty electrical connections on the EGR valve
• Defective control unit software
• Clogged EGR cooler
• Loose or corroded plug connections

What symptoms occur in parallel with fault code P0400?

• Increased emission values: Poor emission values during the main inspection
• Uneven idling: Engine runs unevenly at idle speed
• Loss of power: Reduced engine power, especially at low engine speeds
• Knocking noises: Combustion noises due to increased combustion temperatures
• Increased fuel consumption: Poorer engine efficiency
• Black smoke: Visible soot particles in the exhaust gas

How to fix the problem of trouble code P0400?

• Repairing trouble code P0400 requires a systematic approach to restore proper EGR function.
• Remove the EGR valve, clean thoroughly or replace completely if defective (costs: 150-400 euros)
• Remove soot deposits from the EGR lines and ducts using a special cleaner
• Check EGR position sensor and replace if malfunctioning (costs: 80-200 euros)
• Check all vacuum lines for cracks and leaks and replace defective hoses
• Check electrical connections on the EGR system, clean and replace if corroded
• Clean or replace EGR cooler professionally if blocked (costs: 200-500 euros)
• Update control unit software or have adaptation values reset after repair