A car overheats when the cooling system can't shed enough heat — usually low coolant, a stuck thermostat, failed water pump, blocked radiator, or a head gasket leak. Continuing to drive risks a £2,000+ head gasket repair or a scrapped engine. Pull over immediately, switch off, and call recovery.
Overheating is the one fault that goes from "annoying" to "engine wrecked" in under five minutes. Here's the order to act.
What to do right now
Pull over safely. Switch off engine. Turn heater to MAX heat + fan ON (uses the heater core as a second radiator if engine is still running briefly). Don't open the cap — pressurised coolant at 100°C+ causes serious burns. Call recovery.
Causes ranked by frequency
1) Low coolant — leak somewhere. 2) Stuck thermostat — closed, blocking flow. 3) Failed water pump — coolant not circulating. 4) Cooling fan failure — most common in stop-start traffic. 5) Blocked radiator. 6) Blown head gasket — coolant being burnt or pushed out.
How a mechanic finds it
Pressure test the system to find leaks. Sniff test for combustion gases in coolant (head gasket). Check fan operation. Block-tester chemical for cylinder leakage. Usually £40–£90 diagnostic.
Cost ranges
Top-up + leak fix: £40–£200. Thermostat: £120–£280. Water pump: £250–£600 (often combined with cambelt). Radiator: £180–£500. Head gasket: £900–£2,500+ depending on engine.
Can I drive a short distance if my car is overheating? — No. Even a few miles can warp the cylinder head and turn a £200 fix into a £2,000 one.
How do I top up coolant? — Engine cold only. Use the correct colour/spec coolant for your car (the wrong one corrodes the system). Top to MAX line. Check level daily until you find the leak.
More detail on: Why is my car overheating?
The short answer above covers the usual case. Below is the extra detail a UK driver normally needs to act on it — how to narrow the fault down, what it should cost, and when it is worth paying a garage rather than carrying on. This is general guidance from a working mechanic, not a substitute for someone putting your own car on a ramp.
How to describe the problem so you get a straight answer
Mechanics diagnose faster when they get facts rather than conclusions. Give the make, model, engine size, fuel type, year and mileage; say exactly when the fault happens (cold start, motorway speed, braking, turning, first five minutes); say what changed just before it started; and list any codes, warning lights or recent work. A three-line description in that shape is worth more than a paragraph of theory.
What to do before you spend money
Run a free MOT history check to see whether the same item has been advisoried before — that history often explains a fault in one line. Check whether an open safety recall covers the component, because that repair is free at a franchised dealer. Then get the fault properly diagnosed rather than replacing parts in order of price.
When it is worth going to a main dealer
Three situations justify main-dealer money: an open safety recall (free), a live manufacturer warranty or goodwill claim, and coding or programming work that needs manufacturer-level software. Everything else — servicing, brakes, suspension, clutches, cambelts, diagnostics — an experienced independent will do to the same standard for a good deal less, and using an independent does not invalidate your warranty under UK block exemption rules as long as the correct parts and schedule are used.
Safe to keep driving?
Treat anything affecting brakes, steering, tyres, seat belts or visibility as stop-now, no matter how well the car appears to be driving. Overheating and low oil pressure are also stop-now: the damage curve on both is measured in minutes, not miles. Most other faults will let you drive gently to a garage, but if the car is in limp mode, losing fluid, or the fault changes how it steers or stops, get it recovered instead. Recovery is cheaper than a tow plus a new engine.
What Bob would check first
Start with the cheap, common and visible: fluid levels and their condition, tyre pressures and wear pattern, battery voltage and terminal condition, and every connector and vacuum pipe in the area of the fault. A surprising share of electrical gremlins are a corroded earth or a chafed loom, and a surprising share of running faults are an air leak downstream of the sensor doing the complaining. Only after that does anything get unbolted.
Get an answer on your own car
If you want this looked at properly for your exact vehicle, send the details and Bob will write back — make, model, engine, mileage, when the fault happens and any codes you have. You get a written reply from a qualified UK mechanic with 32 years on the tools, not a chatbot, and your money back if he cannot help.
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