With the engine well warmed up, grasp both heater hoses and feel if they are
both hot; if they're not, water isn't circulating through the heater core.
If the hoses are both hot, the blend door in the heater box is probably not
working correctly; check the cable adjustment.
In the absence of heat it's normal for the defrost to blow cold air since
the A/C compressor is energized in the Defrost mode.
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