My heater works well then suddenly blows
cold then hot again then cold air again.
What is happening??
Check you coolant level cold, the first thing to stop working is the heat if the coolant level starts too drop. If the Coolant is really low then you might consider having a block check test performed just too rule out a blown headgasket, if the coolant is not low then replace the thermostat, it maybe opening too early and not maintaining proper engine temperature.