Radiateur basse température colmaté

The secondary, low-temperature cooling circuit's radiator, restricted so it cannot reject heat. Hybrid, electric and many turbocharged vehicles run two separate coolant circuits, and this one serves the parts that must stay much cooler than the engine — power electronics, the inverter, the charge air cooler, the battery.

Its radiator is usually mounted in front of the main one, which is what makes it vulnerable: it takes the full impact of road debris, leaves and insects, and it is the first thing to block. The blockage is frequently external rather than internal — a matrix packed with debris between the fins, invisible unless you look through it towards a light.

The symptom is a component-specific over-temperature with a normal engine temperature, which is the pattern to recognise: an inverter or charge air temperature climbing under load while the coolant gauge sits where it always does. Check airflow through the matrix before pursuing the pump or the sensors, and be aware that internal restriction from an old or mixed coolant produces the same result with a clean-looking radiator.