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HVAC Issues - "Mode" Door & "Air Mix" Door not working


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Hi guys, just tonight realized that neither the "Mode" Door (directs air between feet/body/defrost) or "Air Mix" Door (controls air temperature) work on my HVAC. I'm getting hot air only and its only coming from the defrost vent. I have ran the HVAC self diagnostics as per the FSM and I'm getting the following error codes:


Sensor error #26 - Air Mix Door Motor (LCU) PBR

Door Motor Switch errors ~31, 32, 34, 35, 36


I have checked the connectors into both the air mix door motor and mode door motor, both are plugged in. 


This is as far as I have went tonight, but it seems I have one of the following;


1. An electrical issue before both door motors (A/C Auto amp or harness)

2. Both motors have somehow failed at the same time.

3. Both doors/motors are frozen due to the recent sustained sub zero temperatures where I live.


Another thing is.. I just got the car back from having my windshield replaced (leaking windshield due to rust where the roof meets the windshield) and this is the first i've noticed the issue. I've been looking at the wiring harness locations to see if this could possibly have been caused by the mechanic changing the windshield. I know the main harness is somewhere near where the windshield meets the firewall area but can only really speculate.


Any thoughts?





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Unlikely that both motors spontaneously quit at the same time. Freezing temps might be a factor if the leaky windshield let enough water into the dash, but I would expect that to either be more obvious (ice all over the place) or clear up when the interior warmed up. And while the timeline suggests the shop may have screwed something up, it doesn't seem like they would've had a reason to touch the dash if all they did was replace the windshield, unless maybe the rust they replaced was along the bottom edge and the dash was in the way.


I haven't looked up the codes. They may specify whether it's an electrical or a mechanical issue. If they don't specify an electrical issue (no circuit open/short), I would check the cabin filter (if equipped) and blower motor for signs of mice. If it sat a while before having the windshield replaced, rodents could've gotten in there and packed enough crap into it to physically obstruct the doors. Long shot, but I've cleaned more mouse mess out of vent systems than I want to think about. (Hopefully the R50 is better sealed against rodents than the WD21.) The smell would likely have tipped you off if this was the case, though.


If the service manual's diagnostics for those codes don't get you anywhere, I would try warming it up first (if you haven't) to rule out ice. If that doesn't work, and especially if the dash has been out and/or the codes do say something's open or shorted, I would check the service manual to see what the wires going to those motors are supposed to do, then unplug and test with a meter. If they're both missing a ground, and the wiring diagram shows that they share a ground, and where that ground goes, then you know where to look for a damaged wire. If neither has any life on any wire, check if there's another plug between them and the head unit or amp or whatever that might've been left undone.

Good luck! Whenever I contemplate troubleshooting the auto HVAC in mine I wonder how much screwing around it would take to convert it to cables.

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Got an update on this..


Done some checks on the wiring between the A/C Auto Amp and the Mode Door Motor.


I'm getting ~12v on the power wire to the motor which mean its getting enough power, continuity on the ground wire, but I'm only getting ~2v on the signal wire and it says i should be getting ~5v. 


I have not checked the Air Mix Door but they share the same signal wire from the A/C Auto Amp.. so it currently looks like the A/C Auto Amp is the problem. This diagnosis makes more sense since both doors seem to have stopped working at the same time.

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The "auto amp" is simply the main AC module, right? If so then I'm chasing a similar issue with my 2003 (no Nav). I randomly get warm air even with the temp all the way down, recirc on and manual mode. I had the whole system tested for leaks and pressure and it's all fine. What seems to be happening is that the AC module tells the blend door to open for a second or two.


I took apart most of the center and lower dash while playing with different temps and settings and all doors appear to be moving fine and fully, well, besides that random "glitch". My plan is to get a used module off eBay to see if the issue goes away but now it sounds like these modules are not very reliable as they age.


Nissan's 90's-00's circuit boards are known for their crappy soldering so perhaps this is just as simple as finding the right resistor and reheat/resolder it? The clock in mine certainly came back to life after doing so, same as my GF's former Sentra's windshield wiper module.

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  • 3 months later...

Apologies I forgot to tie this one up..  I removed the Mode Door Motor (located on right side of driver footwell) and found that the metal rod that is attached was sticking in the track. I just sprayed it with some.lubricant and it's worked fine ever since.

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