Please can someone advise this woman that knows nothing about cars

The "coolant level low stop engine safely" warning appears. This has been happening for quite a while. I top up and after about a 50 kilometre drive the lwarning lught appears again. Sometimes the coolant level has reduced sometimes it doesn't. thermostat has been replaced, pressure test shows no leaks, any pipes showing a slight residue have been replaced.

After these repairs a couple if short trips later and the warning light is back on. With the engine hot the coolant level is a centimetre below the min mark. Let the engine cool and checked again, coolant level a centimetre above the min mark.

No oily residue on my fingers when I dip in the reservoir, no water puddles on the floor.

Please help.

 

well the coolant has to go somewhere :-) . Most common is a small leak in one of the hoses, the radiator seams, the radiator core or the heater core. Also can be a crack in the overflow reservoir. You'd need to look closely again for any effluence stains. Next is a possible head gasket issue where coolant may be finding its way into the combustion chamber. A compression test and HCs in the coolant are the common tests. Any odd smells in the cabin - say when the heat comes on first thing? (leaky heater cores/ hose o-rings can give off a slightly sweet smell). Any odd sweet smell to the exhaust?

Last thought is what happens when you don't top off? does it go substantially below the low fill mark or settle in? Wondering if you have an issue with the level sensor or if you are truly consuming coolant. Did you replace the reservoir cap as well?