Hi, I'm new to the forum and own a 2005 QX56. The A/C works great. 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. I ran the ATC self diagnostics and it did not show any codes. The fan motor works well. The modes change properly to blow at each location. I would think if it's the air mix door not working, the heat would not work on 90 degrees. Can anyone assist?

have a look see at the heat exchanger module in the engine room. has the hvac hoses going to it and then those hoses run into the firewall into the hvac. could be in need of change.

i imagine your coolant and Freon levels are withing spec?

air/mix door could also be stuck/faulty. run the fan on high and switch from the defog setting down to the floor setting. see if you hear any woodpecker like sounds.

you probably wont find any DTCs because this system is mechanical in nature. clearly there is something wrong....

Thanks martronixx. The heater control valve under the hood is working (it's hot on both sides of the valve with the heat on, instead of hot on one side and cold on the other side), so I assume that means it's ok. Coolant and Freon are good (a/c will freeze you when it's hot outside). This started about two years ago, but recently I have heard a low slow popping/pecking sound at times that seems to come from the driver's side, but it's not the same as others have described online and as heard on YouTube.com, it's much slower and does not last long and it only happens once in a while. I will run the fan on high and switch from defog to floor to see if it makes the sound later and reply again. 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'm wondering if it could be an electronic component like a faulty sensor or something.

any woodpecker type noise on these nissan vehicles is most likely a stuck/faulty door. motor could be worn and/or just the door itself.

running the fan on high and changing the modes every 15 seconds or so will weed out potential vent doors. the high velocity air blowing through will make any issues known.

I put the fan on high and switched from defrost to floor. I also switched between all modes, including a auto temp control. There was only a slight low click right after switching modes (even waiting about 15 second, which sounded normal, like a door repositioning). Apparently the hot/cold air mix door only opens partially making luke warm air, or all the way making heat.

Anyone got any instructions on removing the dash? It seems to be the next step to confirm manually.

yes in the service manual for your model year. removing the dash is a VERY involved process!!!!! you will need lots of open space and 3 people at least to help. sensors, air bags, screws to get lost...

ive seen posts on another forum where a guy removed the dash just to replace one of these doors and the photos were brutal...

Thanks. I was told this can be caused: by a fault with an internal air temp sensor. 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. Question is, how do you test the sensor to confirm if it is bad?

if it was the sensor, it should throw a DTC code. you checked the OBD port and found no codes yes?

to test the sensor, you need specialized equipment to test.

the sensor is a thermistor (if memory serves) and you could measure voltage resistance as this would tell the BCM and related modules how to regulate the air handling.

to save headache, just take it in and pay the diagnostic. or you could purchase a new sensor and replace.

did you just purchase this car? if so, is it under a dealer warranty? car lot 30 day (15 day? etc?) warranty?

Thanks for the information. I've owned it for years, so no warranty. It looks like it has three air mix doors. One for driver, passenger and rear. Many people online have said, Air Mix Door, on the Armada/QX56, as though there were only one. If it were an air mix door issue, shouldn't I still get high heat (say on 85 degrees) for the driver, passenger or rear?

lots of "what if's". without being at the car its difficult to give a better opinion.

dash removal may be necessary in this case...

26 DEC 2014 UPDATE. I decided to pay a Nissan dealer about $100 to diagnose the system. After checking, they said it was either the Heater/AC Control assembly (the brain behind the temperature and fan speed knobs) or the entire Heat/AC Box Assembly (includes all the doors for directing air flow) that would have to be replace and either one was going to cost alot. I decided to eliminate the lower cost Heater/AC Control Assembly part number 27500-ZC11B to see if that works first. These are available from $266 and up online. See the following links for reference. If you see the diagram of this its number 27130 in the diagram.
http://www.infinitipartsusa.com/Infinit ... zc11b-.htm
http://www.amazon.com/Infiniti-Genuine- ... B00O4SH9DK
I decided to purchase a used Heater/AC Control from a vehicle that had low or similar mileage, paid $100 plus shipping, installed and it works great. Problem resolved.

good follow up!!

this is one of those instances that are hard to troubleshoot over the internet.

your gum shoe detective work will most definitely save someone else down the road. props to you sir!