2005 GMC Sierra, Air Blows Only From Defroster Vents

This 2005 GMC Sierra came in with the complaint that the air will only flow from the defroster vents, regardless of where it is commanded to. This Sierra has auto a/c controls and no floor mounted center console.

Of course the customer brought the truck in, knowing that I had to lay under the driver’s side of the dash and did not think to vacuum the floor board.

I checked the codes and sure enough there was a code B0263 stored in the HVAC module. Your scan tool has to be able to go into the body systems to see this information. A plain OBD2 scanner just won’t do the job. This repair is so common that you can diagnose it by symptom if you have to. The most common symptom of course is lack of control over the vent output position and normally it will be stuck in the defroster only position. If it is not currently stuck in the defroster position but you are changing the actuator anyways. make sure that you set the vent position to defrost before starting this procedure. You will see why later.

The data lines show an actual mode door position and a commanded mode door position. They do not even come close to matching.

I laid a fender cover on the dirty floor and set the parking brake pedal. The pedal hits me in the shoulder and prevents me from being able to maneuver around.

Now I am ready to get in there and work.

I start by removing the push pin that holds the driver’s floor vent extension in place.

With the push pin released the duct extensions pulls easily from the stationary duct.

The lower screw for the actuator is just above the duct work and cannot be seen. It has to be felt. 5.5 mm head

The second screw can be easily seen as it is at the upper edge of the actuator. Note that it also holds the wiring harness in place.

I have developed a slightly different twist to this repair procedure. I could not take good enough pictures  from under the dash so hopefully I can show you how on the bench and you can transfer that knowledge to the vehicle.

With the two mounting screws removed and the wiring disconnected, use your fingers and thumb as leverage to push the cam/cog from the actuator shaft. The goal is to leave the cam/cog in place on the end of the heater-a/c case and to remove the actuator motor only.

The old assembly on the left and the new one on the right.

The old assembly separated

Taking the new actuator motor and installing it on the old cam/cog. Remember these steps will be done under the dash. Except for removing the new actuator motor from the new cam/cog, that can be done anywhere. I am just doing this on a bench to give you a clearer idea of what has to be done.

The old cam/cog with the new actuator motor installed.  Note the positions of the cam/cog in relation to the motor. Assuming that your actuator has failed in the most common position, blowing out of the defroster vents only, you should not have to do anything other than installing the actuator motor. No indexing of the door shafts to the cam/cog or recalibration of the actuator motors. This is a big time saver.

If you need to purchase one of these actuators please click here.

I recently had to go behind someone that could not properly install the actuator. The actuator and cog were removed from the truck. I installed the cog with the upper peg inserted into the outer slot. I then moved the cog around so that the center/rotating boss dropped into it’s mating position on the HVAC box. Installed the actuator onto the cog and bolted it down. Once connected and activated the actuator turned the cog counter clockwise and indexed the gear tooth lever perfectly. I was a little surprised by this so I tried it several times and every time it aligned properly. When looking at the white geared lever there is one long tooth and when rotated that long tooth should overhang the last black tooth on the cog.  Do Not forget that the actuator needs to be recalibrated in order to work correctly.

44 discussions on “2005 GMC Sierra, Air Blows Only From Defroster Vents”

  1. I have a slightly different issue with my 2005 GMC Sierra but may be the same solution. With a/c on and recirculate selected the air will blow out the dash vents with fan speed on high (5), if I select a lower fan speed (3) or less the blowing air seems will stop blowing out the dash and get lost between all the ventilation with equal but very little air movement out of the dash, floor, and defrost with what seem like air blowing out behind the glovebox. This happens whether air control is on recirculate or fresh but happens more often when recirculation mode is selected. The only correction is turn ignition off and restart.

    1. If it corrects immediately after turning the engine off and restarting I would suspect a faulty mode actuator or a faulty control panel. If it corrects after the vehicle sits for several minutes I would suspect that the evaporator core is icing over and blocking air flow.

  2. If the old actuator has a burnt smell, I would disassemble it and look for heat damage. If present the HVAC control head is likely damaged as well. The doors should be able to be moved manually with then actuator removed. Keep in mind that one may have to be moved some in order for the other one the be freed up to move. They will not stay in place however and would have to be tied in place or just fixed.


  3. My apologies for making a question in a comment section.

    My problem is a little different. Low flow from defrost and no flow (AirCon) from dash. I do get flow from floor vents. Blower speed works, placing mode in recirc causes blower to increase speed. I previously tried recalibrate procedure several times.

    2006 GMC Sierra automatic dual climate control w/ CJ2 rpo.

    The driver side actuator would not rotate so I was unable to set it to defrost prior to removal. My slot on my drive gear is even with the longest mark on the cover and the teeth on the black & white gears were aligned on the last teeth of the arc.

    My actuator has that burned resistor smell.

    With this actuator removed is there a way to manually force air from the dashboard face while awaiting a new actuator? Swinging the 2 levers don’t seem to make air come out the dash vents. Is it possible that I have another problem that is preventing the air from coming from the dash vent?

    I don’t have a scan tool.

    Rgs, Jon

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