warning light guide
Coolant Temperature Warning Light
Your engine coolant has reached a dangerously high temperature or the coolant level has dropped too low. You must pull over and switch off the engine immediately to prevent catastrophic internal damage.
What it looks like: It appears as a red thermometer submerged in liquid, often joined by an audible chime or a rising temperature gauge. Some cars show a blue thermometer when cold, which turns red or flashes red when overheating.
Updated 9 November 2026
Can I keep driving?
Stop driving
An overheating engine can warp the cylinder head or blow the head gasket within minutes, resulting in catastrophic and costly engine failure.
What to do right now
- 1Pull over safely into a car park or hard shoulder as soon as possible and switch off the engine.
- 2Turn the cabin heating to maximum temperature and full fan speed while pulling over to help draw heat away from the engine block.
- 3Pop the bonnet release catch from inside the car, but leave the bonnet closed until steam stops escaping.
- 4Wait at least 30 minutes for the engine to cool completely before inspecting anything under the bonnet.
Likely causes
Most likely first.
- Likely
Coolant leak
A split rubber hose, cracked plastic expansion tank, or stone-punctured radiator allows fluid to escape. Without enough fluid circulating, the engine quickly overheats.
- Likely
Stuck thermostat
The thermostat is a heat-sensitive valve that regulates coolant flow to the radiator. If it jams closed, hot coolant cannot reach the radiator to cool down.
- Possible
Radiator cooling fan failure
The electric fan pulls air through the radiator when the car is stationary or in slow traffic. A blown fuse, faulty fan motor, or bad temperature switch stops the fan from operating.
- Possible
Water pump failure
The water pump pushes coolant throughout the engine passages. An eroded impeller, seized bearing, or snapped drive belt halts circulation entirely.
- Rare
Blown head gasket
A breached gasket allows hot exhaust gases to blast into the cooling passages. This rapidly pressurises the system, blows coolant out of the overflow, and causes intense heat spikes.
What you can check yourself
Park somewhere safe and level, engine off, before you start.
- 1
Check under the car for fluid puddles
Look beneath the engine bay for dripping or pooled liquid, which is typically bright pink, blue, or green with a sweet odour. A sizeable puddle confirms an active external coolant leak.
- 2
Inspect the expansion tank level
Wait until the engine is completely cold, then look at the see-through plastic coolant reservoir. The fluid level should sit comfortably between the minimum and maximum markings without needing to open the cap.
- 3
Look for hose splits or white residue
With the engine cold and switched off, visually check the thick rubber radiator hoses for splits, bulges, or chalky white staining near the hose clips. Stains point to dried coolant from a slow, pressurised seep.
- 4
Test cabin heater air temperature
If the engine seems hot but the dashboard vents blow ice-cold air, coolant is not circulating through the heater matrix. This indicates an airlock, a failed water pump, or a system that has run nearly dry.
Rough cost to fix
Rough estimateTypical prices in Ireland (€) and the UK (£), from an independent garage up to a main dealer. The total bill depends heavily on whether the water pump is driven by the timing belt, which requires hours of labour, compared to an easily accessible external thermostat or hose.
Thermostat replacement
- Do it yourself
- €25–70£20–60 in the UK
- At a garage
- €140–280£120–240 in the UK
Radiator hose or expansion tank replacement
- Do it yourself
- €35–110£30–95 in the UK
- At a garage
- €130–260£110–220 in the UK
Radiator replacement
- Do it yourself
- €80–220£70–190 in the UK
- At a garage
- €250–550£220–480 in the UK
Water pump and timing belt replacement
- Do it yourself
- Garage job
- At a garage
- €350–750£300–650 in the UK
When to go to a garage
- Steam or a sweet burning odour emerges from the engine bay.
- The coolant expansion bottle is bone dry and any added fluid pours straight out underneath.
- The red temperature warning light reappears within five minutes of restarting after cooling down.
- You hear bubbling, gurgling, or metallic knocking sounds from under the bonnet.
Questions drivers ask
Can I drive a short distance to a garage with the coolant light on?
No, you should never attempt to drive with this warning illuminated. Modern aluminium engines can warp their cylinder heads within two to three minutes of running hot. Arranging roadside recovery is substantially cheaper than rebuilding a melted engine.
Can I top up the cooling system with plain tap water in an emergency?
You can use clean tap water to get out of an immediate safety hazard, but only once the engine is cold. Plain water boils at a lower temperature, lacks anti-corrosion additives, and will freeze in winter. You must have the system flushed and refilled with the correct antifreeze mixture at a garage shortly afterwards.
Why is my coolant temperature light illuminated in blue?
A blue coolant symbol indicates that the engine has not yet warmed up to its optimal operating temperature. Many modern vehicles use this blue light instead of a traditional needle gauge on cold mornings. It will turn off on its own after a few minutes of gentle driving and does not indicate a fault.
Will a coolant temperature warning light cause an NCT or MOT fail?
While not a specific dashboard fail item on its own, an active coolant warning will cause the tester to abandon the test. Testers will refuse to perform the mandatory high-rev emissions test on an overheating car due to the risk of engine destruction, and active fluid leaks onto the test lane are an automatic failure.
Test rules here are for the National Car Test in Ireland (NCT) and the MOT in the UK. Elsewhere, check the regulations of your own country or state before going for a vehicle test.
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General guidance for most cars, not a diagnosis of yours. Details differ by make, model and year - check your handbook, and if in doubt, don't drive.