P0400 means the engine control unit has detected that exhaust gas recirculation flow is not within the expected range. The EGR system recirculates a portion of exhaust gas back into the intake to lower combustion temperatures and reduce NOx emissions. A blockage or valve failure disrupts this flow.
P0400 means the engine control unit has detected that exhaust gas recirculation flow is not within the expected range. The EGR system recirculates a portion of exhaust gas back into the intake to lower combustion temperatures and reduce NOx emissions. A blockage or valve failure disrupts this flow.
P0400 is a emissions-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0400 and illuminates the check engine light.
If your vehicle has triggered P0400, you may notice one or more of these symptoms:
Here are the most common causes of P0400, ranked by how likely they are to be the culprit:
Generally yes, for short-term driving. The vehicle is generally safe to drive in the short term, but EGR faults can cause increased NOx emissions, rough idle, and reduced fuel economy. Repair before the next emissions inspection.
Follow these steps to pinpoint the root cause of P0400:
Repair costs for P0400 vary depending on the root cause and your vehicle. Here are typical estimates:
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
P0400 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:
These codes are often seen alongside P0400 or indicate related issues:
The Exhaust Gas Recirculation system routes a small amount of exhaust gas back into the engine intake. This lowers combustion temperatures and significantly reduces nitrogen oxide (NOx) emissions, which are a major component of smog.
Yes, for a short time. The vehicle will run, but you may notice rough idle, knocking under load, and poor fuel economy. The main risk is failing an emissions test and potentially causing increased engine wear from higher combustion temperatures.
On diesel engines, EGR cleaning every 40,000 to 60,000 km is advisable due to higher soot levels. Petrol engines accumulate carbon more slowly, but a cleaning every 80,000 to 100,000 km is good preventive maintenance.
P0400 is a general EGR flow malfunction, meaning the flow is outside the expected range. P0401 specifically means insufficient EGR flow, which is usually caused by a clogged valve or blocked passage. P0400 is less specific about the direction of the fault.
In the EU, blocking or deleting the EGR system is illegal and will cause the vehicle to fail its periodic roadworthiness test (TUV, MOT, ITV, etc.). It also significantly increases NOx emissions. The correct approach is to clean or replace the faulty component.
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