I had the same problem and thankfully it was just the control valve. I took the time to pull the blend door motor off just to see if it was broke. It took more time to check that than it did to replace the control valve. For $15 bucks I would just get the control valve, flush the heater core while you have it pulled apart, then replace the valve. If that doesn't fix the problem then look into the blend door. Save yourself some time and avoid the blend door unless you have to.

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1997 Ford Ranger 2.3 L xlt