Vanne de dérivation du refroidisseur EGR défectueuse

The valve assembly that selects whether recirculated exhaust gas passes through the cooler or straight past it — the actuator, its linkage and usually its position feedback, rather than the flap alone.

The distinction from a flap seized by carbon matters, because the two are diagnosed and priced differently. A seized flap is a deposit problem and can often be cleaned. This entry is the mechanism failing on its own account: a stripped gear or a dead motor in an electric unit, a perished diaphragm or a leaking hose in a vacuum one, a broken return spring, or a position sensor inside the housing that has stopped reporting truthfully.

Which way it fails decides the symptom entirely. Left with the gas going through the cooler, the engine runs rough and smokes when cold and takes longer to warm up. Left with the gas bypassing the cooler, recirculated gas is never cooled at all — combustion temperature rises, nitrogen oxide output climbs, and the emissions codes that follow are consequences rather than separate faults.

Two practical points before anything is ordered. A control circuit that tests good proves the command was sent and nothing more, which is exactly why these valves carry position feedback. And on several engines the valve is integral to the cooler, so what sounds like an actuator is a cooler-and-valve assembly with the coolant system opened to fit it — worth establishing before the job is quoted.