Passenger side has low heat on 2003 Buick Rendezvous?

This problem is very much noticed with vehicle having dual climate control.
On 2003 Buick model,there is no dual climate control.
Even though it has no  dual climate control there are still separate actuators for each side. Most likely the right side actuator has failed on you.

Remove the glove compartment  and you can access and remove this actuator. Then you can open the door manually and see if you have heat. If so then the actuator is bad. If not then you have a problem in the air box itself.

See the diagram below for Actuator.




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For car AC related and Heater related solutions. You can refer the options mentioned below:---

Car AC Blows Warm Air ?

http://technoanswers.blogspot.com/2011/10/car-ac-blows-warm-air.html

How to test AUTO AC pressure?

http://howtobyme.blogspot.com/2011/10/how-to-test-auto-ac-pressure.html

AUTO Air Conditioning Troubleshooting?

http://technoanswers.blogspot.com/2011/10/auto-air-conditioning-troubleshooting.html

AUTO A/C COOLING PROBLEM?

http://technoanswers.blogspot.com/2011/10/auto-ac-cooling-problem.html

Fixing refrigerant leak?

http://howtobyme.blogspot.com/2011/10/how-to-fix-refrigerant-leak.html

How to locate Car AC Refrigerant leak?

http://technoanswers.blogspot.com/2011/10/how-to-locate-car-ac-refrigerant-leak.html

CAR AC INTERMITTENT COOLING?

http://technoanswers.blogspot.com/2011/10/car-ac-intermittent-cooling.html

What causes INTERMITTENT COOLING on Car AC system?

http://technoanswers.blogspot.com/2011/10/what-causes-intermittent-cooling-on-car.html

Car AC making Noise?

http://technoanswers.blogspot.com/2011/10/car-ac-making-noise.html

BAD ODOR ON CAR AIR CONDITIONER?

http://technoanswers.blogspot.com/2011/10/bad-odor-on-car-air-conditioner.html

How to Recharge Car's Air Conditioner

http://repairhelpcenter.blogspot.com/2011/10/how-to-recharge-cars-air-conditioner.html