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A radiator or heat exchanger can no longer shed the heat it was sized for — blocked on the outside by debris, or restricted on the inside by deposits.

The two kinds of blockage look identical on a gauge and need completely different work, so telling them apart comes first. Outside is leaves, insects, road film, mud or a plastic bag against the core — and on modern vehicles, a stack of heat exchangers mounted one behind another, where the debris collects invisibly between them. Inside is scale, casting sand, corrosion products or the remains of a stop-leak additive, which narrow the tubes.

The outside case is the commoner one and the cheaper one by a wide margin, and it is genuinely worth looking at before anything else: a torch through the core from the front tells you a great deal for nothing. The inside case usually announces itself differently — a radiator with blocked passages runs cold in patches, which can be felt by hand across the core once warm, where a healthy one is evenly hot.

The symptom pattern is the diagnostic gift. A restricted radiator copes at speed and fails in traffic, because airflow at 70 mph hides a great deal that idling does not. So "it only overheats when I stop" points here or at the fan, and "it overheats on the motorway" points somewhere else entirely — at flow, at the pump, or at a thermostat.

On an electrified vehicle this matters in more places than it used to: the traction battery and the power electronics have their own heat exchangers, often stacked in the same airflow, and one blocked core can limit charging rate or drive power on a car whose engine cooling is perfect.