My AC unit for a 2003 RSX-S does not blow cold air. The heater works fine and the fan is working good as well.

There is a relay that powers the blower motor circuit . Left I/P fuse box , do you know where that is ? The relay is IGN 3 relay .
Fuse Block- I/P - Left
LF of the passenger compartment, behind the kick panel
IGN 3 RELAY
--
IGN3, HVAC CNTRL MODULE
IGN 3
10A
HVAC CNTRL MODULE, TC SW., HTD SEAT SW.
Does your car have Manual or automatic climate control ?

Make sure your fan clutch is working. No fan, no cooling of the condenser, no heat dissipation, no cold air. Start the truck and make sure you feel air movement, a lot.

To troubleshoot this types of problem,click the link below :----http://howtobyme.blogspot.in/2012/02/heater-blows-only-from-defrost.html
--------Go through the procedure to confirm the issue.

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Engine Overheats and heater blowing cold?

http://howtobyme.blogspot.com/2011/11/engine-overheats-and-heater-blowing.html

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Heat blows cold on drivers side and hot on passenger side?

http://schematicsdiagram.blogspot.com/2011/12/heat-blows-cold-on-drivers-side-and-hot.html

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No heat on car? http://technoanswers.blogspot.in/2012/02/no-heat-on-oldsmobile-silhouette.html

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Heater blows intermittently and temp gauge goes erratic? http://technoanswers.blogspot.com/2012/01/heater-blows-intermittently-and-temp.html------------Heater blows cold? http://schematicsdiagram.blogspot.in/2012/02/heater-blows-cold.html
---------Car heater blowing warm, but not hot air? http://schematicsdiagram.blogspot.com/2011/12/car-heater-blowing-warm-but-not-hot-air.html-------- Heater/AC blower motor not working?

http://technoanswers.blogspot.in/2012/02/heaterac-blower-motor-not-working-on.html

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A/C, heater, defroster fan, not working? http://technoanswers.blogspot.in/2012/02/ac-heater-defroster-fan-not-working.html Heater problem? http://technoanswers.blogspot.in/2012/02/heater-problem-on-isuzu-trooper.html---------

This will help.Thanks.

For this types of problem,i suggest you to check this help links:----Car heater blowing warm, but not hot air?http://schematicsdiagram.blogspot.com/2011/12/car-heater-blowing-warm-but-not-hot-air.html
---------A/C, heater, defroster fan, not working?http://technoanswers.blogspot.in/2012/02/ac-heater-defroster-fan-not-working.html
--------Heater problem?http://technoanswers.blogspot.in/2012/02/heater-problem-on-isuzu-trooper.html
---------Engine Overheats and heater blowing cold?http://howtobyme.blogspot.com/2011/11/engine-overheats-and-heater-blowing.html
-----------Heat blows cold on drivers side and hot on passenger side?http://schematicsdiagram.blogspot.com/2011/12/heat-blows-cold-on-drivers-side-and-hot.html----------------This will help.Thanks.

your ac compressor is freezing up try not putting it on cold all the way. put thermostat towards the red jus a lil but still on the cold side to see if that helps

Is the engine temperature at the proper point?
Since the AC works, I assume the fan is blowing. This means I would look at the temperature sensors and control unit next. Try varying the temperature setting way up to see if there is a point where it starts working.

Heater control valve faulty,
Air lock/blockage to heater matrix,
control unit voltage sender.
Check the above in this order. I assume you are operating heater control functions correctly and the heater display is showing no unusual infor.?
hope this helps. good luck.

There is only one computer. Either the filter still has plastic wrap on it, or there is not air going through squirel cage on fan motor to the vents.

check cooling fan operation while ac on.If fan is working proper,will probably have stuck exspantion valve.

Try spinning the knob real fast in either direction. Sounds stupid but it worked for me.