Bypassklappe des AGR-Kühlers festgefressen

The flap that routes recirculated gas around the cooler, jammed by carbon on its spindle. The bypass exists because cold EGR is not always wanted — on a cold engine, hot gas routed past the cooler helps the engine and catalyst reach temperature faster; once warm, the gas should go through the cooler.

Which direction it seizes in decides what breaks. Stuck open, recirculated gas is never cooled, so combustion temperature rises, nitrogen oxide output climbs, and on a vehicle with reductant dosing the SCR system is asked to handle more than it was designed for — so downstream emissions codes follow and are consequences rather than separate faults. Stuck closed, the engine warms slowly and runs rough when cold.

It sits directly in the soot stream, so binding is ordinary rather than exceptional, and a flap that has seized was sticking for a long time first — usually setting intermittent codes that were cleared.

Cleaning is often a genuine repair where the actuator is sound and the spindle unworn, which is worth asking about because the assembly is expensive. Be aware the fault may be thermal rather than mechanical: the module often infers flap position from temperature, so a faulty sensor produces the same code with a free-moving flap.