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The recirculation valve held open when it should be closed, so exhaust gas enters the intake at times the engine cannot tolerate it. Of the two ways this valve fails, open is the one the driver notices — stuck closed costs emissions and nothing else the driver can feel.

Exhaust gas at idle and on light throttle displaces the oxygen the engine needs. The result is a rough, hunting idle, stalling at rest, a hesitation pulling away, and black smoke — symptoms that read as fuelling and send the diagnosis to injectors. An engine that idles badly and pulls cleanly at speed is describing this, because at higher load the recirculated fraction is a smaller proportion of total flow.

It also reduces torque under load, which is why it appears on insufficient-torque codes. Carbon holding the valve off its seat is the usual mechanism — the seat and the shaft foul together — so check the valve closes fully by hand as well as reading its position feedback, since a valve reporting closed while held a fraction off its seat is exactly this fault.