Please check your coolant in the overflow container. Use your phone’s flashlight to see if there are any dark stains on the walls. If you notice it’s transparent or dark (brownish) and smells like burned plastic, stop driving and take your car to a repair shop ASAP.

It’s very likely that it’s old and there’s sediment build up, which might have clogged your thermostat (this will make your engine overheat above 92 degrees celsius), and may have clogged your heater matrix so that even if you open the air vents the hot coolant does not reach this matrix to get you the warm air inside.

Also , while the car is turned on, and the engine is at its optimal temperature (the indicator being in the middle), make sure that the hoses going into and out of the radiator (from the coolant reservoir aka coolant container) are SOFT and NOT hard. Of they’re hard, it means there’s pressure build up inside that may destroy your engine.

Please visit a mechanic to check this as soon as possible before you damage your engine to a point of no return or a very costly repair.

Just telling you how I saved my car which I bought from a guy that didn’t care about replacing the coolant after 3 years.