Car overheats in traffic but not on the motorway
Overheating specifically at low speed or standstill, that clears once you're moving again, points to a cooling problem that depends on airflow through the radiator rather than overall coolant capacity - at speed, ram air through the radiator covers for a weak point that stop-and-go traffic exposes. The fix is almost always the fan or the radiator's front-on airflow path.
Urgency
Stop driving
Possible causes, most likely first
- Cooling fan not switching on, or running too slowly
At speed, airflow through the grille does the fan's job for free; in traffic, that airflow disappears and a weak or non-working fan can't make up the difference. Watch the fan through the grille at idle with the AC on to see if it runs at all and how fast. - Debris blocking airflow through the radiator or AC condenser
Leaves, bugs, or light debris packed into the fins in front of the radiator reduce airflow enough to matter only when there's no ram air to compensate, exactly the traffic scenario. Visually inspect the radiator face through the front grille or bumper for a packed-in layer of debris. - Low coolant level reducing the system's heat buffer
A slightly low system has less thermal mass to absorb heat during the lower-flow conditions of idling in traffic, even if a slow leak isn't otherwise causing problems at speed. Check the expansion tank level cold. - Faulty fan control relay or module
A relay or module that fails to trigger the fan at the right temperature threshold, specifically when road speed can no longer mask the issue, leaves the engine without its only cooling source at a standstill. Check for the fan starting on manual command via a scan tool even if it doesn't start itself. - Worn viscous fan clutch (belt-driven mechanical fan)
On vehicles with a mechanically driven fan, a worn clutch doesn't engage fully at low engine speed, which is exactly when idling in traffic needs it most. Check if the fan spins freely with very little resistance by hand when the engine is off and cool. - Trapped air reducing coolant flow at low engine speed
At idle, pump flow is at its weakest, and a trapped air pocket that doesn't matter at higher flow rates can block proper circulation specifically then. Check for gurgling from the expansion tank or heater at idle.
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What you can check yourself
- Watch the fan through the grille at idle with the engine warm and AC on to see if and how fast it spins.
- Check the radiator face for a packed layer of leaves, bugs, or debris.
- Check the coolant level in the expansion tank when cold.
- Note the exact speed/conditions where the temperature starts climbing versus where it stabilises.