The car will usually drive normally, and this is often found only at an emissions test. Ask whether the EGR passages were checked and not just the valve — replacing the valve alone is the classic way this fault comes back.
Less exhaust gas is reaching the intake than the module asked for. This is by a wide margin the most common EGR fault, and on most engines the cause is not the valve but the passage behind it: exhaust gas carries soot, the passage narrows over years, and eventually the restriction is enough that a fully open valve still cannot flow the commanded quantity.
That distinction matters commercially. A valve is a part; a blocked passage is labour. Replacing the valve on an engine whose passages are packed solid produces a car that passes for a few thousand miles and returns with the same code, which is a common and avoidable outcome. Before condemning the valve, check whether it moves freely and whether the port behind it is open — on many engines the intake-side port can be seen with the valve removed.
The other pattern worth knowing is the vacuum-operated system with a healthy valve and no vacuum reaching it. Applying vacuum directly to the valve at idle should stall or badly disturb the engine. If it does, the valve and passage are fine and the fault is upstream in the solenoid or its hoses.
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P0401 — EGR flow insufficient — reads like a broken valve, and the valve is what gets replaced. But the monitor is measuring the *effect* of EGR flow (a change in manifold pressure...
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