Why does my A/C blower blow vent or hot air then eventually cold air? on 2003 Mercury Grand Marquis

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2003 Mercury Grand Marquis about six months ago the A/C would work as it should and then occasionally would start out blowing warm air and then after about 5 min. it would blow cold air. Then it became sporadic until now it blows warm air for an extended period of time and then eventually will blow cold air. What should I do to resolve the issue?
(2) Answers
The HVAC control heads give a bunch of trouble on these vehicles. Have a mechanic with a 'mighty vac' tool check it out! That's all that is needed to find this problem.
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