AGR-Kühlventil defekt

The valve directing recirculated exhaust either through the cooler or around it, faulty so the routing is wrong. It is not the recirculation valve — that one meters how much gas flows, while this one decides whether that gas is cooled first, and the two are separately coded.

The bypass exists for cold running. Cooled recirculation lowers combustion temperature and oxides of nitrogen, but during warm-up the engine needs its heat, so the valve routes gas around the cooler until temperature is reached. Stuck in bypass, the engine recirculates hot gas permanently and oxides of nitrogen rise; stuck in cooled, warm-up is slowed and condensation forms in a cooler that never gets hot.

That condensation is the part worth knowing. Exhaust gas cooled below its dew point deposits acidic water inside the cooler, which corrodes it — so a valve stuck in the cooled position eventually produces a leaking cooler as a second, more expensive fault. Check the valve's actual position against its command with the engine cold and again warm, since a valve that works in one state and not the other is common here.