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Air trapped in the cooling system, blocking circulation. Air collects at the high points and forms a lock that coolant cannot push past, so flow stops in that branch while the rest of the system circulates normally.

The symptoms are contradictory, which is the signature: overheating with a full expansion tank, a temperature gauge that swings, and a heater that blows cold while the engine runs hot — because the matrix is often the highest point and the first to airlock. Modern engines are hard to bleed by gravity, with several high points and long runs, so many require a vacuum fill or a specific bleed procedure. A system that airlocks again after being bled has a leak drawing air in, or a failed pressure cap letting it in as it cools.