P0400: Exhaust Gas Recirculation A Flow
What to do
Usually driveable. Expect a failed emissions test and, on some engines, hesitation or a rough idle. Worth attending to before a test rather than after.
What this code means
The exhaust gas recirculation system is not moving the quantity of gas the module commanded, and the fault has not resolved into the more specific insufficient or excessive codes. EGR exists to lower combustion temperature by returning a measured amount of inert exhaust gas to the intake, which suppresses the formation of oxides of nitrogen. Because the gas is inert, it displaces oxygen without contributing to combustion — so too much of it causes rough running and stalling, and too little causes higher combustion temperatures and detonation under load.
The generic flow code is worth treating as a starting point rather than a diagnosis. On most systems the module does not measure flow directly: it infers it, from a position sensor on the valve, from a differential pressure sensor across a metering orifice, or from the change in manifold pressure when the valve is commanded open. Establishing which method the engine uses tells you which sensor is worth trusting, and that decides the whole rest of the diagnosis.
Likely causes
Start here
- Carbon-blocked EGR passage · moderate
- Stuck or seized EGR valve · moderate
- Faulty EGR position sensor · easy
Less common
- Failed EGR vacuum solenoid · easy
- Disconnected or leaking vacuum hose · easy
- Corroded or loose connector · easy
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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