How to know that the liquid is really decreasing
Check the level with the engine cooled down, on a flat surface and according to the MIN and MAX marks in the car manual. When warm, the liquid expands in volume, so the results at different temperatures can be misleading. If the level drops again after correct filling, a warning appears or the tank is empty, this is not just a routine check and the cooling system needs to be checked.
Where coolant can leak
The leak can be on a hose or its connection, radiator, expansion tank and cap, thermostat body or water pump. Fluid can also leak through the heater or other part of the cooling circuit of a particular engine. The list of possible locations is wide, so it is not possible to reliably determine the faulty part just by how fast the level drops.
Why doesn't there need to be a puddle under the car
A small leak sometimes appears only after heating and pressurizing the system. The liquid can hit the hot part of the engine and evaporate or remain trapped in the cover under the engine. Suspicions may be raised by dried paint traces, a sweet smell, moisture in the engine compartment or fogging of the windows associated with an odor in the cabin, but even their absence does not rule out a leak.
What to do when the engine overheats
If the temperature gauge rises into the red zone or the cooling warning comes on, stop safely and follow the instructions for the specific car. Continuing to drive can seriously damage the engine. Allow the engine to cool and do not open the cap of a hot expansion tank: the system is under pressure and hot liquid or steam can cause scalding. When in doubt, choose towing.
How to top up fluid safely
Use only fluid with the specification prescribed by the vehicle manufacturer; color alone does not determine suitability. Do not mix different types without verifying compatibility and do not fill above the MAX mark. Replenishment can help address low levels, but will not correct the cause of repeated depletion. After a major leak or overheating, it is safer to have the car inspected first.
How to find the cause in the service
The inspection begins with an inspection of the tank, lid, hoses, connections, radiator and the water pump area. According to the manifestation of the defect, a pressure test of the cooling system is carried out, checking the tightness and function of the fan or thermostat; in case of suspected internal leakage, also other targeted measurements. The goal is to find the specific cause before parts start changing.
Every vehicle and fault is different. This article provides general guidance; an inspection of the specific vehicle determines the correct procedure.