If your vehicle is showing vehicle overheats only when idling or in stop-and-go traffic, here are the most likely causes and the parts you'll need to fix each one.
The electric cooling fan pulls air through the radiator when the vehicle is stationary. If the fan motor burns out or its relay fails, there is no airflow at idle, causing rapid overheating that resolves once you start moving again.
On vehicles with engine-driven mechanical fans, a fluid-filled clutch engages the fan when the radiator gets hot. When the clutch leaks or wears out, the fan slips and spins too slowly at idle to cool the engine.
The engine computer relies on temperature sensors to know when to turn on the electric cooling fan. A faulty sensor or switch will fail to trigger the fan when the engine gets hot at idle.
Low coolant or trapped air pockets reduce the system's ability to absorb heat. At low engine RPMs, the water pump doesn't spin fast enough to overcome this lack of fluid, leading to overheating. This is a very common installation error if the system wasn't properly bled after recent cooling system work.
Over time, the impeller blades inside the water pump can corrode or wear down. At highway speeds, the pump spins fast enough to circulate coolant adequately, but at idle, the flow becomes too weak to prevent overheating.
Leaves, dirt, or plastic bags trapped between the A/C condenser and the radiator severely restrict airflow. The cooling fan struggles to pull enough air through the blockage at idle, though ram-air at highway speeds might still be sufficient.