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Q:

heater goofed up on 2000 Mercury Mountaineer

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my heater blows warm air then cold air,it keeps alternating when i have the heat on
(3) Answers
(1) Comments
You could have an issue with your automatic temperature control if it is so equipped. A temperature sensor could be at fault, or a temperature blend door issue could be present (but not likely for this concern...). Lastly, you could have a thermostat issue - sticking open intermittnetly.
it also has airconditioner is alternating when turned on ,sometimes blows cold air sometimes regular air,put new thermastat in and had all fluids changed,oil and coolant and transmission fluid it heater and air conditioner still changes air.heater will work for while then blows cold.air conditioer blows cold then without touching any of the controls it blows hot air.
Please refer to the original answer; could have a temperature blend door actuator failure. A proper diagnosis is in order.