2003 Chevy Trailblazer Vortec. Battery replacement. I tried pulling the HVACB and HVAC1 fuses.  
Left for a minute and started the engine left for 2 minutes.  Turned off engine.  Turned it back on
and tried to see if it would work and it didn't.
Dash vents will not work.  Manual system.

I would disconnect the negative battery cable for 10 minutes, then do the procedure you have
tried again. If that does not help, you may have a problem in the a/c control head. Replacing
battery can sometimes do bad things to electronics, especially if not done properly.
Re-learning the door positions can also be done with the dealer scanner. This re-home is better
than just removing power from the system. And, you may end up there anyway to have the
system properly diagnosed

Chevy Trailblazer LS 2006
Warning lights = engine,
oil light
Computer codes = P0449
Repairs:  changed spark plugs, changed oil and filter. Looking around Internet for a picture of a part.
Question = need to know where the vent solenoids are located.  That's one of the possible problem.
Answer: P0449 on your Trailblazer is usually the EVAP vent solenoid. But have had a few that were
a wiring problem under the hood of the
4.2L V6. To be sure you need to have a scanner that can
control the solenoid on and off. Then have a
test light in the 2 wires at the solenoid and make sure it
lights up. If not, you have a wiring problem.

Under the hood in the main engine harness that is below the computer (PCM) is where I have found
a few broken. Either the power or the ground circuit to the vent solenoid. If the vent is bad, it is
located at the rear of the fuel tank on a bracket. It has one electrical connector and a large vent
tube on it. Sort of between the drive shaft and tank.
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2003 Trailblazer Dash Vents Not Working