P0400: Exhaust Gas Recirculation Flow Malfunction
Quick Answer
Safe to drive (short-term)
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.
- Severity
- Moderate · Emissions
- Estimated repair cost
- From €50 to €1400+, varies by vehicle
€50 or €1400, depending on your car.
- Replace DPFE or EGR pressure sensor€50–€180
- Replace EGR cooler€550–€1400
A scan shows whether P0400 is really stored on your car, which codes sit next to it and how urgent each one is.
A scan needs a Bluetooth OBD2 adapter (Bluetooth LE on iPhone), about €25-60; the built-in demo runs without one. See which adapter to buy
What does P0400 mean?
P0400 is a emissions-related diagnostic trouble code classified as moderate severity.
Where it is on the car
- Engine
- Exhaust manifold
- EGR cooler (diesels and many newer petrols)
- EGR valve and position sensor
- Intake manifold
Too little or too much flow is often carbon in the valve or its passages, or a blocked cooler, not only a failed valve.
What are the symptoms of P0400?
- Check engine light on
- Rough or unstable idle
- Engine knocking or pinging under load
- Reduced fuel economy
- Possible deficiency at an emissions inspection while the warning light is on
- Hesitation during acceleration
What causes P0400?
Carbon-clogged EGR valve
: high
Blocked EGR passages or cooler
: high
Failed EGR valve (stuck open or closed)
: medium
Faulty EGR position sensor or pressure feedback sensor
: medium
Vacuum supply issue to vacuum-operated EGR valve
: low
Is it safe to drive with P0400?
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.
How do you diagnose P0400?
- Check for companion EGR codes (P0401, P0402, P0403) that may help narrow down whether the issue is insufficient flow, excessive flow, or an electrical fault
- Remove the EGR valve and visually inspect for carbon buildup. A heavily clogged valve is the most common cause and can be cleaned with carburettor cleaner
- With the EGR valve removed, inspect the intake and exhaust passages for carbon blockage. Use a flashlight and probe to check passage diameter
- Test the EGR valve: apply vacuum to a vacuum-operated valve, or on an electronic valve command it open and closed with manufacturer-level software (a generic OBD2 reader cannot run this actuator test). It should open and close smoothly
- If the valve operates correctly, compare commanded and actual EGR in live data (standard OBD2 values on many cars) and check the DPFE or EGR position sensor against the manufacturer specification
How much does P0400 cost to fix?
| Repair | Parts Cost | Labor Cost | Total Estimate | DIY Difficulty |
|---|---|---|---|---|
| Clean EGR valve and passages | €5–€30 | €60–€200 | €65–€230 | Moderate |
| Replace EGR valve | €60–€350 | €60–€250 | €120–€600 | Moderate |
| Replace EGR cooler | €300–€800 | €250–€600 | €550–€1400 | Professional |
| Replace DPFE or EGR pressure sensor | €20–€80 | €30–€100 | €50–€180 | Easy |
Prices estimated as of October 2026. Costs vary by region, vehicle, and shop.
Full repair guides
Engines known for this code
P0400 on specific vehicles
Volkswagen
Volkswagen Golf
2003-2024
Ross-Tech lists the EGR valve, its wiring and connectors, and the EGR system as a whole as the possible causes of P0400 on VW group diesels.
Source: wiki.ross-tech.com
P0400 on Ford
Ford Focus
Opel
Opel Astra
2004-2010
The Astra H (2004-2010) is offered with the 1.7 CDTi diesel (Z17DTL, Z17DTH, Z17DTJ and Z17DTR).
Source: en.wikipedia.org
Peugeot
Peugeot 308
2007-2024
The Pierburg EGR valve listed for PSA 1.6 HDi engines is electrically operated.
Source: andreadisparts.com
Related diagnostic codes
Common questions about P0400
- What does the EGR system do?
- 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.
- Can I drive with P0400?
- Yes, for a short time. The vehicle will run, but you may notice rough idle, knocking under load, and poor fuel economy. The main risks are higher nitrogen oxide output and wear from higher combustion temperatures, and with the warning light on the fault can be recorded as a deficiency at a periodic inspection.
- How often does the EGR valve need cleaning?
- Diesel engines soot the EGR up far faster than petrol engines, so a diesel reaches the point of needing a clean much sooner. How quickly depends on how the car is driven: short trips and stop-start use load it fastest.
- What is the difference between P0400 and P0401?
- 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.
- Is it OK to delete or block the EGR valve?
- No. Blocking or deleting the EGR system breaks the emissions type approval the car was registered under, which is why it is prohibited across the EU, and an inspector who finds it has grounds to reject the vehicle, with the checks and the penalties set by each country. It also significantly increases NOx emissions. The correct approach is to clean or replace the faulty component.
Read our detailed guides
Sources
- wiki.ross-tech.com wiki.ross-tech.com/wiki/index.php/16784/P0400/001024
- andreadisparts.com andreadisparts.com/en/products-1/egr-valves/pierburg-egr-valve-psa-1-4-1-6-hdi
- en.wikipedia.org en.wikipedia.org/wiki/Opel_Astra