Low Coolant Warning Keeps Coming On? Here's How to Find the Leak
Topping up your coolant once and never thinking about it again is fine. Topping it up every week because the warning light keeps coming back? That's a problem. Coolant doesn't evaporate. If the level keeps dropping, it's going somewhere - and you need to find out where before your engine pays the price.
This guide covers every place coolant can disappear to, how to find the source, and what each repair is likely to cost.
Why Low Coolant Is More Serious Than Low Washer Fluid
Coolant (antifreeze) keeps your engine from overheating. It absorbs heat from the engine block and carries it to the radiator where it gets cooled down before cycling back through. If the level drops enough, the engine overheats. Run it hot long enough and you're looking at a warped cylinder head or a cracked engine block - repairs that cost thousands.
A low coolant light that comes on once after not checking it in a year is probably fine.External Leaks
External leaks are where coolant is physically dripping or seeping out of the cooling system somewhere you can see. These are the more straightforward ones to find.
Radiator Hoses
The two large hoses connecting the radiator to the engine are under pressure and get very hot. Over time, the rubber hardens and cracks, or the clamps that hold them on corrode and loosen. Check both ends of each hose - the connection points are where most leaks start.
What to look for: Staining or discolouration near the hose connections, a crusty residue (dried coolant tends to leave a whitish or rust-coloured deposit), or actual drips under the front of the car.
Cost to fix: A new hose is $20-$60. Fitting is another $50-$150 at a shop. Straightforward job.
Radiator Itself
Radiators can develop small cracks or pinhole leaks, especially in older cars or after an impact. Even a small rock can punch through an aluminium radiator fin. Most radiator leaks are at the end tanks (the plastic side sections on modern radiators) which crack with age.
What to look for: Coolant dripping from the front of the car, near where the radiator sits. Sometimes you can see a stain on the radiator core itself.
Cost to fix: A replacement radiator is $150-$500 depending on the car, plus labour. Some shops offer radiator repairs for small cracks, but replacement is usually more reliable long-term.
Water Pump
The water pump circulates coolant through the engine. It's driven by the timing belt or a separate accessory belt. When the seal on the pump fails, coolant leaks out around the pump housing - usually a weep hole designed to tell you it's time for replacement.
What to look for: Coolant dripping or staining from the front-lower area of the engine. On timing belt-driven pumps, you often can't see it without removing the covers. A mechanic can check with a pressure test.
Cost to fix: $300-$800 depending on whether it's a timing belt-driven pump (the belt usually gets replaced at the same time, which adds to the cost but saves money overall).
Heater Core
The heater core is a small radiator inside the dashboard that heats the cabin. When it leaks, coolant either drips under the passenger side dashboard or turns up as a sweet-smelling fog on the inside of your windscreen.
What to look for: A sweet smell inside the car when the heater is running, foggy windscreen that doesn't clear properly, or damp carpet on the passenger side floor.
Cost to fix: This is one of the more expensive coolant leaks because the dashboard often needs to come out to access it. Expect $500-$1,500 depending on the car.
Freeze Plugs
Freeze plugs (also called core plugs) are metal discs pressed into holes in the engine block. They're designed to pop out if the coolant freezes and expands, protecting the block. Over many years they can corrode and start to weep. Less common but worth knowing about on higher-mileage cars.
Cost to fix: $150-$400 per plug depending on location - some are easy to reach, others require partial engine removal.
Internal Leaks
Internal leaks are where coolant is getting into the engine oil or combustion chamber rather than dripping on the ground. These are more serious.
Head Gasket Failure
The head gasket seals between the cylinder head and the engine block. When it fails, coolant can leak into the combustion chamber (where it gets burned and exits as white smoke) or into the oil passages (where it mixes with oil and turns it milky).
What to look for:
- White sweet-smelling smoke from the exhaust, especially after startup
- Oil that looks milky or light brown on the dipstick (looks like a milkshake)
- Bubbles in the coolant reservoir when the engine is running
- Engine overheating even after topping up coolant
A head gasket failure won't show up as an external puddle under the car, which is why coolant that keeps disappearing without any visible drips should raise this suspicion.
Cost to fix: $1,000-$2,500 for the head gasket replacement itself. If the head is warped from overheating, add $300-$800 for resurfacing (or more for replacement). This is one repair worth comparing quotes on.
Cracked Cylinder Head or Block
Worse than a head gasket. Usually caused by severe overheating at some point. A crack in the cylinder head or block can leak coolant internally with the same symptoms as head gasket failure but is harder and more expensive to fix.
If the head gasket has already been replaced and the overheating or coolant loss continues, a crack is likely.
Faulty Sensor
Sometimes the coolant level sensor itself fails and reports low coolant even when the level is fine. Before assuming a leak, check the actual coolant level in the reservoir and the overflow tank. If the level looks correct and stable over several days, a faulty sensor is worth investigating.
Cost to fix: Coolant level sensors are usually $20-$80 for the part, plus an hour of labour.
Using the Right Coolant
If you're topping up frequently while tracking down a leak, make sure you're using the right coolant for your car. Mixing coolant types (green, orange, red, blue) can cause the additives to react and form sludge that blocks passages and can accelerate corrosion.
Check your owner's manual or ask TorqueBot - the correct coolant type varies significantly between manufacturers and even between models from the same brand.
A mechanic can attach a pressure tester to the coolant system and pressurise it while the engine is cold. Any leak will show itself when it can't hold pressure. This is the best way to find small leaks that aren't obvious to the eye and is usually a free or low-cost diagnostic step.
If you're seeing coolant disappear but can't find where, ask for a cooling system pressure test before spending money on guesswork.
What to Do Right Now
If the warning light just came on:
- Pull over safely and let the engine cool before opening the bonnet
- Once cool, check the coolant reservoir level (the plastic tank connected to the radiator - don't open the radiator cap on a hot engine)
- Top up to the MAX line with the correct coolant if needed
- Check under the car for any puddles or drips
- Watch whether it comes back down over the next few days
If the light comes back on within a week, book it in for a cooling system pressure test. Don't keep topping up and hoping for the best - coolant loss that progresses to overheating is a much bigger repair bill.
Ask TorqueBot to Narrow It Down
The right coolant type, the most common failure points on your specific engine, and what a realistic repair should cost in your area - TorqueBot can give you all of that based on your car's year, make, and model. Different engines have well-known weak points (BMW N54 water pump, VW 1.8T coolant flanges, Subaru EJ25 head gaskets) and knowing what to look for first saves time and money.