Use where the engine overheats—idle, stop-and-go, highway, or long pull—to separate fan trouble from thermostat, coolant-flow, and restriction problems faster.
That pattern points hardest at airflow. Cooling fans, fan relays, fan modules, or blocked airflow at the radiator and condenser rise to the top because road speed is temporarily doing the fan's job for it.
Highway overheating pushes coolant-flow capacity higher than fan logic. Thermostat behavior, water-pump performance, low coolant, trapped air, and radiator restriction deserve more attention when speed alone does not rescue temperature.
When the A/C gets warm at idle shortly before the gauge climbs, weak fan airflow becomes much more believable. Engine cooling and condenser cooling often fail together in that pattern.
A recent coolant repair, thermostat replacement, or partial drain-and-fill can leave trapped air behind. An older neglected system is more likely to have scale, weak caps, clogged radiator passages, or pump wear.
First confirm coolant level and whether the system can hold pressure. Second verify fan behavior hot and with A/C on. Third judge where the overheating happens most consistently. That order usually prevents random thermostat and sensor replacement.