P0400 EGR A Flow

Investigate soon

The ECU has logged a fault on P0400. The definition below is the exact wording the standard uses for this code.

Can you drive it? The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.

The definition

P0400: EGR A Flow.

P0400 is a powertrain code (the engine or gearbox), defined by the SAE standard - it means the same thing on any make of car, and any generic OBD-II reader will decode it.

What to check first

Cheapest and fastest checks first:

Read the freeze-frame data stored with the fault - engine speed, load and temperature at the moment it logged tell you whether this happens cold, hot or under load.
Check the connector at the part for corrosion, and that it is fully seated.
Clear the code and see whether it returns, and under what conditions.

What actually causes this on a VAG

Exhaust gas recirculation feeds a measured amount of exhaust back into the inlet to keep combustion temperatures - and NOx emissions - down. The ECU doesn't measure that flow directly: it works it out, on VAG diesels typically from the mass airflow (MAF) reading, and compares what it got with what it asked for. P0400 is the general "EGR flow isn't what I expected" complaint, and you'll often find more specific EGR codes stored alongside it, which are usually the more useful place to start. Symptoms range from nothing noticeable to a lumpy idle, hesitation, smoke or reduced power.

Most likely causes, in order

Carbon clogging the EGR valve and inlet passages. This is the classic on VAG diesels, particularly cars used mainly for short, cold journeys. The valve, the pipework and the inlet manifold silt up with soot until the valve can't move properly or the passages are choked. Removing and inspecting the valve tells you straight away.
A tired mass airflow sensor. Because the ECU infers EGR flow from the MAF signal on these engines, a MAF that has drifted with age throws EGR flow codes even though the EGR system is fine. Compare the actual airflow figure against the specified figure under load in live data before you spend a penny on EGR parts.
Vacuum or actuator problems on the valve. On older vacuum-operated systems, a split hose, a weak vacuum pump or a sticking control solenoid (often labelled N18) means the valve never opens as commanded. On electric valves, the motor or gearing can seize. An actuator test with a scan tool, watching whether the valve actually moves, settles it.
Blocked EGR cooler or its bypass flap. The cooler and its flap soot up in the same way as the valve and can restrict flow even with a healthy valve. Usually found once the valve has been ruled out and the system is opened up.

Worth knowing: Check the MAF live data before buying an EGR valve. A drifting MAF is a well-known cause of EGR flow codes on these cars, and it is a far cheaper and easier part than the EGR valve it often gets blamed on.

Reading the code is the easy part

A code names the circuit that complained. What tells you why is watching the sensor behind it while the engine runs — and that is where a code reader stops and a diagnostic tool starts.

VAGPULSE reads every module in a VW, Audi, Skoda or SEAT — not just the engine — shows the live measuring blocks behind this fault, gives you the freeze-frame conditions it was logged under, and clears it once you have fixed it. It also does what a generic OBD reader cannot on a VAG: service resets, coding, adaptations, and Stage 1–3 flashing with your original file saved first.

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Common questions

What does P0400 mean?

P0400 means EGR A Flow. It is logged when the ECU sees this circuit outside its expected operating window.

Can I drive with P0400?

The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.