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Heater blows cold air and after 20 miles or so it will start getting warm. It then starts blowing cold again. It goes back and forth like this while running.
Sounds like a classic thermostat issue.
However, if the engine is hot enough, it could be a temperature blend issue inside the dash.
Q&A > Ford > 1997 Ford Explorer
Q&A > Ford > 1996 Ford Explorer
Q&A > Ford > 2004 Ford Explorer