On my sons 2000 chevy silverado 4.8 liter....the heater works fine....but when i switch control to defrost....only cold air blows out defrost vents....any help would be appreciated.

1: not enough liquid in the system (air in the heater core)
2: inoperative heater valve not allowing hot liquid into the core
3: hot/cold blend door in the heater box stuck on cold
4: clogged heater core
A clogged heater core will have one hot hose and one significantly cooler hose at the firewall connection; easily tested by hand. a good core will have both hoses very hot to the touch when the heater is on full blast.
Make sure that the system is full and at operating temperature, and the heater controls set on full heat. Feel the hoses at the firewall; if they are both too hot to touch then the problem is in the heater box or controls.

As per your problem mentioned. There are few solved help links to troubleshoot these problems. Go through the list and Click the link below:----

2000 Toyota Avalon: No heat on Passenger Side?

http://whatisbyme.blogspot.in/2012/05/2000-toyota-avalon-no-heat-on-passenger.html

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No heat on Oldsmobile Silhouette?

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

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Heater blows cold?

http://schematicsdiagram.blogspot.in/2012/02/heater-blows-cold.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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Hot air on the driver's side vents only?

http://repairhelpcenter.blogspot.com/2012/01/hot-air-on-drivers-side-vents-only.html

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Temperature gauge does not move and no heat when heater is ON?

http://technoanswers.blogspot.com/2012/01/temperature-gauge-does-not-move-and-no.html

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1992 Toyota Camry: A/C Control light Flashing and A/C not Blowing Cold?

http://technoanswers.blogspot.in/2012/04/1992-toyota-camry-ac-control-light.html

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No heat, blower blows only cold?

http://technoanswers.blogspot.in/2012/02/no-heatblower-blows-only-cold.html
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Ford F150 Ac only blows through defrost vents?

http://technoanswers.blogspot.com/2012/01/forf-f150-ac-only-blows-through-defrost.html

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Dodge A/C come out of defrost vents only?

http://technoanswers.blogspot.com/2012/01/dodge-ac-come-out-of-defrost-vents-only.html

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Ac vacuum line on dodge cummins?

http://technoanswers.blogspot.com/2012/01/ac-vacuum-line-on-dodge-cummins.html

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Ford ranger AC blows cold through defrost vents instead on cabin vents?

http://technoanswers.blogspot.com/2012/01/ford-ranger-ac-blows-cold-through.html

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2005 Chrysler PT Cruiser: A/C Blows Warm?

http://whoisbyme.blogspot.in/2012/06/2005-chrysler-p8t-cruiser-ac-blows-warm.html

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2009 Mercury Grand Marquis: A/C blows Cold From Defrost Vent Only?

http://whoisbyme.blogspot.in/2012/06/2009-mercury-grand-marquis-ac-blows.html

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AC not blowing through Vents on 2007 Chevy Trailblazer?

http://schematicsdiagram.blogspot.in/2012/04/ac-not-blowing-through-vents-on-2007.html

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Passenger side temperature is warm air out of dash vents?

http://schematicsdiagram.blogspot.in/2012/04/passenger-side-temperature-is-warm-air.html

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These will help.

Thanks.

Heater failure is designed to fail in defrost mode. Check for leaking or dislodged vac hoses.

If it is blowing hot air out of the defroster and not out of the heater vents then the door that directs the heat from the defroster isn't working properly it is blocking the heat from going to the heater vents. Also check the cold air door. It sounds like it isn't closing all the way to shut off the cold air and not letting the hot air pass.

Had the same thing happen to my truck, the blen door actuator is under the console on passenger side, but the heater core is going to be clogged if you dont have any heat coming through, but a/c works..

Blowing out of the defrost is a vacuum problem. You have a leak between the motor and the heater controls. Most likely needs to have the A/C system charged up.

Had the same problem with my Mazda6 awhile back. Took it to the dealer and they found the radiator wasn't holding pressure. They replaced the whole radiator under warranty (WHEW!). Not sure how to check for that kind of thing, but it's a start.

most american cars force the defroster to blow cold air, this is not a problem, it does this by design.

Check your Heater Core

i had the same problem with my 2000 chevy truck!! i took both hoses off engine and flushed the heater core with water and air frome air compresser,added new coolent and it works perfect!!!! so try flushing your system............