If defrost is on it could be your compressor going bad. See what happens when you leave off defrost or disconnect your compressor. It only controls defrost and air conditioning. I had the same problem
The 2003 Chevrolet Trailblazers has two heater cores. One operates the front area of the vehicle while the other heats the rear of the vehicle.
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Value of a 2003 Trailblazer w130,000 miles
It could be coming from your heater core, may need to repair or replace it.
Where is the input aux for a 2003 trailblazer
how do you set the clock on a 2003 chevrolet trailblazer
Where is the neutral safety switch on a 2003 trailblazer?
yes
The blower resistor for a 2003 Trailblazer is located under the passenger side of the dash.
No
location for 2003 chevy trailblazer coolant temperature sensor
How do I reset the service engine light on a 2003 Chevy Trailblazer?