2002 Chevrolet Trailblazer-Air Will Not Blow Out Of The Floor Vents For Heat

Rodney: My HVAC blows hot and cold but all vents are open except driver and passenger feet vents. I bought a mode actuator but that didn’t solve the problem. I recalibrated the hvac many times via snap on scan tool. Pressing the mode button doesn’t change the vents direction. I have two codes b0159 and b3761. Help please!

Sparky: Does the display on the HVAC control head move through all of the positions with the pressing of the mode button? I understand that the airflow does not change, I just want to make sure there is not a problem with the control head.

The code B0159 is for a fault with the ambient or outside air temperature sensor. The B3761 code is for a problem with a mode door actuator. I would start with removing and reinstalling the HVAC B fuse in the rear fuse box to initiate a recalibration and not use the scan tool. Start the engine and do not touch anything. Allow the system to run for at least two minutes but no more than four. Turn the ignition off for at least ten seconds but no more than thirty. Restart the engine and check a/c operation and look for codes. My assumption is that both will return. The next step will be to make sure the correct actuator has been replaced. There are several on that side of the dash. Please provide the number off of the old actuator. If at all possible take a picture of the installed location and upload it here. Will your scan tool read HVAC data? Perform bidirectional control of actuators?

Rodney: The display moves through all of the positions. I bought a new mode actuator from The-Electric-Connection.com and according to the picture I changed the one in the middle. I did the recalibration the way you said to but no change. I still have the codes. I have a snap on Solus pro. This is the label on the actuator that I removed. Part number 52489426.


Sparky: When I looked the other day in an aftermarket source B3761 was just listed as a mode control fault. Through further research in GM SI, I found that the B3761 code is specifically related to the defrost actuator. That actuator will be located above and towards the firewall compared to the mode actuator. In the Trailblazers and Envoys the mode/vent function is split between the mode and defrost actuators.

The B0159 code is for the ambient temperature sensor located behind the grille. It has two wires: light green/black wire and a brown wire. You should be able to look at data and see the voltage and temperature data. Standard temp sensor testing. Check readings of disconnected versus jumped together. Should change between a nominal 0 to 5 volts. The key may have to be cycled between tests to get the data to update.

I have never used the Snap On Solus Pro so I will need some information from you. Can you go into Special Functions or Active Tests in order to command individual actuator movement? Can you view data stream and see actual and commanded steps? Can you see command status/voltage?

Rodney: Can you send the part number for the defrost actuator please?

Sparky: 15-72972 is the ACDelco part number.

Rodney: I tried a new air temperature sensor, so far seems to be working because the code B0159 is gone. I’m now left with the B3761. The defrost actuator counts on the scanner is at zero not moving at all.

Only code present.


Rodney: I installed the new defrost actuator and recalibrated the system. Problem is gone. Question: Why does it say drivers blend door actuator (15-72972) and not defrost actuator on everybody’s sites?

Sparky: The primary use for it is as a blend door actuator but it is used for the other applications as well. There is a very good chance that if you were to look at the numbers on the labels on your actuators (Defrost & Driver’s Temp) the numbers will match.

2 discussions on “2002 Chevrolet Trailblazer-Air Will Not Blow Out Of The Floor Vents For Heat”

    1. I would suspect that the battery has been disconnected recently. If so the throttle body will need to be cleaned. Do Not spray cleaner into the throttle body. Spray it on a rag and wipe the plate and bore surfaces clean.

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