Luftstrom zum Kühler behindert

The radiator's face blocked so air cannot pass through it, which is a cooling failure with nothing wrong in the cooling system. The radiator only works if air moves through the matrix, and everything the road throws forward accumulates exactly there.

Leaves, insects, plastic bags and road debris pack between the fins, and the blockage is genuinely invisible from the front because the outer face still looks like a radiator. Look through it towards a light — that is the test, and a matrix that passes little light passes little air. On vehicles with stacked coolers the debris sits between the layers, where it cannot be seen at all without separating them.

The symptom is overheating that depends on airflow rather than on load. Temperature climbs in slow traffic and settles at speed, which is the opposite of a circulation fault and points at heat rejection. Aftermarket additions matter here too — a badge bar, an auxiliary lamp, or a bug screen across the grille reduces flow measurably, and so does front-end accident repair that has left a deflector or undertray missing.