Coolant keeps disappearing but there’s no leak: where it goes
Updated 23 July 2026 · OBDCode UK editorial
Can you keep driving?
Do not drive with the cold level below minimum or any rising-temperature warning. Never remove the cap hot. Small top-ups do not make ongoing loss safe; a minor leak can suddenly empty the system.
Match what the car is doing
| What you notice | Most likely | How to separate it |
|---|---|---|
| Sweet smell or damp passenger carpet | Heater-matrix leak | Check screen mist, carpet and cooling-system pressure. |
| Pink/white crust but no wet patch | Hot leak evaporating on engine or radiator | Inspect hose joints, thermostat housing and water pump after a pressure test. |
| Hoses go hard quickly from cold | Combustion gas entering coolant | Cooling-system gas test and cylinder tests are needed. |
| White exhaust plus falling level | Possible internal coolant leak | Separate normal condensation from persistent sweet-smelling vapour after warm-up. |
| Loss around cap/overflow | Weak cap or over-pressurisation | Test the cap and system pressure; do not assume the cap caused the pressure. |
Checks to do before buying parts
- Photograph the cold level at the same temperature and parking angle for several days.
- Inspect undertrays as well as the driveway; they can hold or spread a leak.
- Request a cold and hot pressure test plus cap test.
- If internal loss is suspected, read our head-gasket cost guide before approving strip-down.
Typical UK costs
| Likely job | Typical fitted price |
|---|---|
| Pressure/cap test | £40–£100 |
| Hose, cap or small housing repair | £30–£300 |
| Radiator | £250–£500 |
| Heater matrix | £300–£1,000+ |
| Head gasket | £700–£1,700+ |
Indicative 2026 independent-garage prices for mainstream cars; London, prestige, SUV and seized-part jobs can cost more. Diagnose first.
MOT relevance
Fluid leaking so that it is likely to harm the environment or pose a risk can be a defect. The larger risk is breakdown or engine damage before test day.
Frequently asked questions
Is it normal to top up coolant every few months?
No regular top-up schedule is expected on a sealed system. A one-off small correction after service is different from a level that repeatedly falls.
Should I use head-gasket sealer?
Sealer can obstruct small cooling passages and may hide rather than repair the fault. Confirm where the coolant is going first.
Sources and further reading
General guidance for UK drivers, not professional advice. MOT rules follow the DVSA inspection manual (2026 edition).