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My heater venst dont switch away from defrost or floor on my 2005 gmc envoy 4.2 i have a vacuum dtc any ideas on what line it will be?

Posted by Ty Swisse on


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Jeff Armer

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I have a vacuum dtc ? What is the DTC #

Posted on Dec 01, 2020

  • Jeff Armer
    Jeff Armer Dec 01, 2020

    The following DTCs are for the HVAC door actuators:

    • B0248 is for the defrost actuator.

    • B0263 is for the mode actuator.

    • B0408 is for the left air temperature actuator.

    • B0418 is for the right air temperature actuator.

    plus these are not vacuum code's , these actuators are controlled electrically . The problem would be with the mode door actuator , this controls where the air blows from , floor , vent or defrost .
    The HVAC control module controls the HVAC door actuators to regulate the airflow through the HVAC system. Each actuator consists of a stepper motor, a logic circuit, and a potentiometer. The potentiometer inside the door actuator allows the module to monitor the current position of the actuator drive shaft. The logic circuit inside the actuators receives control signals from the HVAC control module and controls the internal stepper motor. When a door positional change is required, the HVAC control module calculates a commanded door position. The module compares the commanded door position to the actual door position and determines the needed direction of motor rotation. The module applies a signal voltage to the door control circuit that is an input to the internal logic circuit of the door actuator. A 5-volt signal increases the door position. A 0-volt signal decreases the door position. When the commanded door position is equal to the actual door position, the HVAC control module sends a 2.5-volt signal to the door actuator and motor rotation stops. The ignition 3 voltage circuit provides source voltage to the logic circuit. The module provides ground to the actuator logic circuit through the low reference circuit.

    lot's of videos on youtube on replacing these actuators .



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SOURCE: heater blower not working on 2005 Envoy

the blower is on a different fuse than the heat deforst one if the resistor is faulty the fan would run on speed 5 only so still could be fuse or motor check for power to main motor under glove box ??

Posted on Feb 04, 2009



  • 116 Answers

SOURCE: Vacuum line leak

sounds like cruise control cutout switch but dont give me bad rap if i didnt have enough info.

Posted on May 20, 2009


Colin Stickland

  • 22485 Answers

SOURCE: a/c and heater only blow air out of defrost

either a vacumn pipe is broken or the ends split so its not passing a vacumn to open the flaps,where they are and how to get to them i cant help because i have never worked on one as they are not here in spain.check the engine end first though .Look for some small rubber pipes running from the inlet manifold

Posted on May 30, 2009


Harvey N Tawatao

  • 3854 Answers

SOURCE: 2005 GMC Envoy Locked in 4WD all the time

If your luck it's going to be a $15.00 vacuum switch on the transfer case that takes about 15 min to replace, if it's the electronic solenoid to the transfer case then it's going to be cost a little more and take a little longer. If your try switching from 4WD to 2WD and hear the solenoid working by hearing the cluck's when it engages and disengage then it's going to be the vacuum switch on top of the transfer case on the drivers side. Auto Zone Sell's them for $15.00 and they will have the step by step instructions to replacing the switch. They will also have the transfer case solenoid  too, if you don't hear the solenoid. Check the fuses, the fuse may have gone bad. Good luck and hope this helps.

Posted on Jun 19, 2009


  • 8 Answers

SOURCE: 97 Lumina...Switch stuck on defrost vent and

I had same problem . There is a vacuum resovoir tank located behind the left front splash shield . The vacuum line behind the battery goes to it . I removed two 5/32 screws just in front of the tire that hold the splash shield to the fender , carefully flex the shield away from the fender and take a look . I found the vacuum line had come loose , plugged it back on and everything works . Hope this helps

Posted on Feb 28, 2010

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