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The heat exchanger cooling engine oil, failed either by blocking or by leaking internally between the oil and coolant circuits. The internal leak is the serious one, because the two fluids mix — and which way they mix depends on which pressure is higher.

Oil in the coolant shows as a sludge in the expansion tank; coolant in the oil gives a milky emulsion and destroys bearings quickly. Either finding on an engine with an oil cooler should point here before the head gasket, which is the far more expensive conclusion and the one usually reached first.

Blocked rather than leaking is the quieter failure. Oil temperature climbs under sustained load, its viscosity falls below what the bearings need, and the module reports oil temperature or pressure — with a cooler that looks perfect. Compare oil temperature across a long run under load, and check the cooler's own airflow or coolant flow, since a blocked cooler and a cooler with nothing flowing through it behave identically.