Car Overheats While Driving: Causes and What To Do

What To Do Right Now

If the temperature gauge is climbing into the red or a red temperature warning light is on, work through this in order.

1. Switch off the air conditioning. The AC condenser sits in front of the radiator and adds a significant heat load. Turning it off helps immediately.
2. Turn the cabin heater to maximum heat, fan on full. This feels wrong on a hot day and it is deeply unpleasant, but the heater matrix is a second radiator. It genuinely pulls heat out of the engine and can buy you the distance needed to reach somewhere safe.
3. Pull over safely and switch the engine off. Do not brake hard or stop somewhere dangerous — but stop at the first safe opportunity.
4. Open the bonnet if you can do so safely, to let heat escape. Watch for steam and stand clear of it.
5. Wait. Do not open anything. The system is pressurised and will stay dangerously hot for a long time.
6. Do not restart to “just get home”. This is the decision that turns a $300 repair into an engine replacement.

When It Overheats Tells You Where To Look

Before opening anything, note the pattern. It narrows the list considerably.

1. Low Coolant or a Leak

The most common cause by a wide margin, and the easiest to confirm. Coolant is not consumed in normal operation — if the level has dropped, it went somewhere.

Look for: a puddle under the car (coolant is usually bright green, orange, pink or blue and feels slippery), white crusty residue around hose joints, a sweet smell after a drive, or steam from under the bonnet.

Common leak points: radiator hoses and their clamps, the radiator itself (especially the plastic end tanks on modern cars), the water pump weep hole, the thermostat housing, the heater core — which leaks into the passenger footwell rather than onto the road — and the expansion tank.

Fix: $100 – $500 for a hose or clamp; $400 – $1,000 for a radiator. If you can find no external leak at all but coolant keeps vanishing, suspect the head gasket and read white smoke from the exhaust.

2. Thermostat Stuck Closed

A thermostat stuck closed blocks coolant from reaching the radiator entirely, so the engine temperature climbs steadily with nowhere for the heat to go.

Recognise it by: rapid overheating that starts within 10 – 15 minutes of setting off, with the upper radiator hose staying suspiciously cool while the engine gets hot. That temperature difference is the giveaway — if the engine is hot and the top hose is not, coolant is not circulating.

Fix: $150 – $400. Note that this is the opposite failure to P0128, where the thermostat sticks open and the engine never warms up.

3. Failed Water Pump

The water pump circulates coolant through the engine. When it fails — a corroded impeller, a worn bearing, or a slipping drive — flow drops and heat stops moving.

Recognise it by: a whining or grinding noise from the front of the engine that changes with engine speed, coolant weeping from the small hole on the underside of the pump housing, or visible play in the pulley. On engines where the pump is driven by the timing belt, a failing pump can take the belt with it, which is far more serious.

Fix: $400 – $900, and considerably more where the pump sits behind the timing belt. If the timing belt is anywhere near due, do both at once — the labour overlaps almost entirely.

4. Cooling Fan Not Working

At road speed, airflow through the radiator does the work. At a standstill, the fan does it. A dead fan therefore produces a very distinctive pattern: fine on the open road, temperature climbing in traffic.

Recognise it by: exactly that pattern, plus air conditioning that blows warm at idle and cold once moving. With the engine hot and idling, the fan should audibly cut in.

Check: the fan fuse and relay first — both are cheap and both fail. Then the fan motor itself, and the coolant temperature sensor that tells it when to run.

Fix: $30 for a relay, $250 – $600 for a fan assembly.

5. Blocked or Clogged Radiator

Radiators block in two directions. Externally, the fins clog with leaves, insects, dirt and road debris, which you can often see by looking through the front grille. Internally, corrosion and old degraded coolant leave deposits that restrict the tubes — a common outcome when coolant has never been changed or when plain water has been used to top up.

Recognise it by: gradual worsening over months, overheating mainly at speed or under load, and sometimes cool patches you can feel across the radiator face with an infrared thermometer.

Fix: external cleaning is free. A chemical flush is $100 – $200. Replacement is $400 – $1,000.

6. Blown Head Gasket

This is both a cause and a consequence of overheating, which is what makes it so important to stop early. Overheating warps the head, the gasket fails, and then the failed gasket causes more overheating.

Recognise it by: thick white smoke from the exhaust that does not clear once warm, a creamy beige sludge on the oil filler cap or dipstick, bubbles rising continuously in the expansion tank while the engine idles, coolant disappearing with no external leak, or overheating that returns almost immediately after a top-up.

Confirm it: a combustion leak test — a chemical block tester held over the coolant that changes colour if exhaust gases are present — is inexpensive and definitive.

Fix: $1,200 – $3,000, and more if the head needs machining. On an older car this is often the point where the repair exceeds the value of the vehicle.

7. Other Causes Worth Knowing

• A collapsed lower radiator hose. The reinforcing spring inside degrades and the hose collapses under suction at high rpm, choking flow at exactly the moment it is needed most.
• A faulty radiator cap. The cap maintains system pressure, which raises the boiling point of the coolant. A cap that no longer holds pressure lets coolant boil at a much lower temperature. It is one of the cheapest parts on the car.
• An air pocket after a coolant change. If the system was not bled properly, trapped air blocks circulation. A very common cause of overheating that begins immediately after cooling system work.
• The wrong coolant, or plain water. Water alone boils sooner, offers no corrosion protection and accelerates internal blockages.

Can You Keep Driving?

If you are genuinely stranded and must move the car a short distance, let it cool completely first, top up the coolant if you have some, run with the heater on full, and stop the moment the gauge climbs again. Move it in short stages rather than one continuous run. This is damage limitation, not a repair.

When to See a Mechanic

Call for recovery rather than driving if any of these apply:

• Steam is coming from under the bonnet.
• The gauge is in the red, or a red temperature light is lit.
• You can see white smoke from the exhaust, or the oil looks milky.
• Coolant is visibly pouring or dripping out.
• The engine has started knocking, has lost power, or has stalled.

When you describe the fault, say when it overheats — in traffic, at speed, uphill, or all the time — and whether you have been topping up coolant. Those two facts eliminate most of the list before anyone opens the bonnet. See also our cooling system section for the individual components.

Frequently Asked Questions