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The charge air cooler restricted internally or unable to reject heat, so air reaches the engine hotter and at lower pressure than intended. It sits between the turbocharger and the engine, so it degrades boost and charge temperature together — which is why it appears on intake temperature, boost and underboost codes at once.

Oil is the usual internal blockage. A turbocharger with worn seals passes oil into the charge air, and it pools in the intercooler's lower tank and coats the internal fins. So a blocked intercooler is a symptom of the turbocharger, and finding oil in it during diagnosis is a stronger indication of turbo seal wear than anything measured at the turbo itself.

Externally it blocks like any radiator — it sits at the front and packs with debris, leaves and insects. Check the charge air temperature rise across it under load: air entering hot and leaving barely cooler is a cooler that is not working, whatever its appearance, and a restriction shows as a pressure drop between its inlet and outlet.