What should I do if the coolant-temperature warning comes on?

Stop safely and let the engine cool. Do not open a hot pressurised cooling system, and do not continue driving an overheating engine until the cause is understood.

In relation to “What should I do if the coolant-temperature warning comes on?”, the pattern of a fault matters: cold or hot, under braking or acceleration, only at speed or only in traffic. Those details can save a lot of diagnostic guesswork.

For “What should I do if the coolant-temperature warning comes on?”, do not remove a hot expansion-tank or radiator cap. Let the system cool fully and investigate coolant level, fan operation, leaks and circulation before further driving.

For “What should I do if the coolant-temperature warning comes on?”, the mechanical principles here are not unique to Northern Ireland, but the roadworthiness consequence is relevant to an NI driver. Use the vehicle handbook for model-specific instructions and the attached official sources — nidirect - The Highway Code for Northern Ireland, nidirect - MOT and vehicle testing, and GOV.UK - Vehicle recalls and faults — for legal or testing context. A dashboard symbol can mean different things between manufacturers, so the exact symbol and behaviour matter.

Practical next step for “What should I do if the coolant-temperature warning comes on?”: record the warning or symptom, check the handbook and basic safe-to-check items, and arrange diagnosis if the cause is not obvious. Stop driving if there is a red warning, loss of braking/steering, severe overheating, heavy smoke, major fluid loss or any other sign the car may be unsafe.