P0400

Exhaust Gas Recirculation Flow Malfunction

P0400 is a generic OBD-II powertrain diagnostic trouble code: Exhaust Gas Recirculation Flow Malfunction. It is logged by the engine control unit when the egr monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P0400
Group
Powertrain
System
EGR
Severity
Warning (MIL on)
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What P0400 means

Code P0400 — Exhaust Gas Recirculation (EGR) Flow Malfunction — is set when the powertrain control module (PCM) detects that exhaust gas flow through the EGR system is outside the expected range. The EGR system intentionally routes a metered quantity of exhaust gas back into the intake manifold to dilute the incoming air/fuel charge, lowering peak combustion temperatures below approximately 2,800 °F and thereby reducing the formation of nitrogen oxides (NOx), a regulated smog-forming pollutant.

The fault can reflect either too little flow (a stuck-closed valve, clogged passage, or failed control signal) or too much flow (a stuck-open valve), both of which the PCM distinguishes via feedback sensors such as the DPFE (Differential Pressure Feedback EGR) sensor on Ford applications, EGR temperature sensors, or MAP sensor response during commanded valve actuation. Because the root cause spans mechanical, vacuum, electrical, and software sub-systems, diagnosis requires a methodical approach rather than immediate component replacement.

While the vehicle is often still driveable when P0400 is stored, ignoring the fault can lead to increased NOx emissions, failed emissions inspections, engine knock from elevated combustion temperatures under load, and progressively worsening fuel economy. Prompt diagnosis is recommended, especially if secondary symptoms such as rough idle, hesitation, or detonation are present.

Common causes

Most-frequently reported root causes when P0400 is logged.

  • 1
    Faulty or stuck EGR valve (open or closed)
  • 2
    Carbon buildup blocking EGR passages or ports
  • 3
    Cracked, disconnected, or collapsed vacuum lines to EGR valve or control solenoid
  • 4
    Defective EGR control solenoid
  • 5
    Failed or out-of-range EGR feedback sensor (DPFE, EGR temperature, or pressure sensor)
  • 6
    Damaged or corroded wiring and connectors in the EGR control circuit
  • 7
    Exhaust leak upstream of the EGR pickup point reducing available exhaust pressure
  • 8
    PCM software fault or calibration issue requiring an update

Symptoms drivers notice

Check Engine Light (MIL) illuminated
Rough idle or engine hesitation, especially at light throttle
Engine knock or detonation under acceleration
Hard starting or stalling (if EGR valve is stuck open at idle)
Reduced power and poor acceleration
Decreased fuel economy
Failed emissions or smog test

How to diagnose P0400

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool, record all stored DTCs and freeze-frame data to identify operating conditions at fault set
  2. 2
    Perform a visual inspection of EGR valve, vacuum lines, hoses, and wiring harness for obvious damage, cracks, or carbon deposits
  3. 3
    Command the EGR valve open via the scan tool's bi-directional controls and observe MAP sensor or RPM response — a healthy system will cause idle to roughen or drop
  4. 4
    Test EGR vacuum lines for leaks using a hand vacuum pump; verify the valve diaphragm holds vacuum on vacuum-operated systems
  5. 5
    Check DPFE or EGR temperature/pressure sensor voltage and compare against specification; replace if out of range
  6. 6
    Inspect and clean EGR passages and intake ports of carbon buildup; use appropriate chemical cleaner or mechanical cleaning
  7. 7
    Clear codes and complete multiple drive cycles to confirm the repair; verify no return of P0400 or related NOx codes

Vehicles where we've handled P0400

Platforms in our catalogue with confirmed P0400 coverage.

AUDI A4 20D
AUDI A6
2015
AUDI A7 30D
AUDI A6 30D
2015
AUDI A7

Related powertrain codes

Frequently asked questions

Can I drive with a P0400 code?

In most cases the vehicle remains driveable with P0400 stored, but you may notice rough idling, hesitation, or engine knock under load. Continued driving with elevated combustion temperatures can cause long-term engine damage, and the vehicle will likely fail an emissions test. Diagnosis within a reasonable timeframe is recommended.

Is P0400 the same as a bad EGR valve?

Not necessarily. While a faulty EGR valve is one common cause, P0400 can also be triggered by clogged passages, failed vacuum lines, a defective DPFE or EGR pressure sensor, a bad control solenoid, or wiring issues. Replacing the valve without diagnosing the root cause often does not resolve the fault.

How is P0400 different from P0401 and P0402?

P0400 is the generic 'flow malfunction' code indicating the PCM detected an EGR flow problem without specifying direction. P0401 means insufficient EGR flow (too little), and P0402 means excessive EGR flow (too much). P0400 may appear alone or alongside P0401/P0402 depending on the vehicle's diagnostic strategy.

Does P0400 always fail an emissions test?

Yes, in most jurisdictions an active P0400 with the MIL illuminated will result in an automatic emissions test failure, regardless of whether tailpipe NOx levels are measured. Many test programs also fail a vehicle with any stored, non-cleared DTC accompanied by an illuminated MIL.

Disabling P0400 in software

RaceTune can permanently disable P0400 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

ECUs with a P0400 disable in our catalogue

Confirmed coverage from our recipe database — we support many more families. Upload your file and our identifier will match it automatically.

  • Bosch EDC17C74 verified 1 software version
  • Bosch EDC17CP44 verified 1 software version

Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.

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