2005 Infiniti QX Heat only works on 90 degrees

Tiny
904CAL
  • MEMBER
  • 2005 INFINITI QX
  • V8
  • 4WD
  • AUTOMATIC
  • 100,000 MILES
Hello. I own a 2005 QX56. The A/C works great in summer. When the weather turns cold, the heat is mostly luke warm which means you freeze with it set on 85 degrees. If I set it on 90 degrees, then the heat works very well, but can only stand it for few minutes on that setting. Then go back to 85 degrees and you freeze again. The fan motor works well. Coolant and Freon are good (a/c will freeze you when it's hot outside). The heater control valve under the hood is working (it feels hot on both sides of the valve with the heat on, instead of hot on one side and cold on the other side). It seems like it would most likely be the air mix door actuator, but kind of doesn't make sense because the heat works normal when all the way up on 90 degrees, so I was wondering if it could be an electronic component like a faulty sensor or something. I ran the ATC self-diagnostics 2-3 times and it did not show any codes in the ambient temperature display. However, I was told if there was "If you have an air temp sensor that is sensing the interior is hotter than it actually is when you set the heat, anything but 90° it will keep cool like this. When set on 90° this bypasses the temp sensors and it blows full hot no matter what the temperature is inside the vehicle. I have seen this type of fault before and it is usually a temp sensor failure causing this needing replaced". I'm trying to avoid paying a dealer to remove the dash, etc if possible. Can anyone offer any recommendations to help pin point the problem so I can decide how to get it fixed? Any help would be greatly appreciated.
Saturday, November 1st, 2014 AT 9:23 AM

2 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,119 POSTS
The blend air door is what determines how much heat you get. My guess is that it is sticking or the actuator isn't working properly.
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Saturday, November 1st, 2014 AT 7:41 PM
Tiny
904CAL
  • MEMBER
  • 2 POSTS
Thanks. I found a diagram showing how the air mix door is separate from the driver, passenger and rear air output for each zone. Then I removed the main control panel (with HVAC, Stereo and Nav controls knobs), then partially loosened and lifted the CD changer to look for the air mix door linkage. It can be seen fairly well with a flashlight. Starting with temp on lowest setting at 60 degrees, the next setting up goes to 65, and could see a good amount of movement, then stepped the temp up one degree at a time and could see slight movements on almost every step up to 85 degrees. The final step up goes from 85 to 90 degrees, and again I could see a good amount of movement, plus (since the fan was set on high) I could hear the air movement changing significantly. So from 60 to 65 and 85 to 90 is where the most noticeable amount of movement and sound of air changing direction occurred. At this point, it seems like the air mix door and motor are working properly unless I'm missing something. Also, since the motor and linkage are moving at each temp adjustment, it seems the in vehicle temp sensor is working as the system knows to keep moving the door. I'm starting to think there is not enough coolant flowing through the heater core, although the line going into it is very hot and it heats well IF set on 90 degrees. Any suggestions?
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Sunday, November 2nd, 2014 AT 7:00 AM

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