2015 Xterra heater not working

[Chuckl]

I have a problem with my 2015 Xterra that started at the end of winter last year. My heater quit working. At first I could cycle the temp knob back and forth and then the heater would start working. I checked the heater hoses going into the firewall and they were hot. Then I thought there might be air in the heater core. I was going to get new coolant and a flush anyhow so I took it to the shop hoping I would solve two problems. When I got it back heater was working. I live in Mississippi and sometimes at the end of winter it is cold in the morning and hot in the afternoon so I would be going back and forth between heat and air. For the next week or so everything was OK because I was staying on heat. Then a day came up where I put it on air and the next day heat would not work. Since the days were starting to get warmer I didn’t worry about the heat until now.

I think I have checked the simple stuff. I don’t think there is air in the hoses or heater core and it is hot going into the firewall. I could switch the temp knob back and forth and after a bunch of times it may start working. I don’t know exactly what happens when you turn the temp knob. I don’t know what module might be involved that adjust the temp. I know something has to happen to mix the air. Any ideas on where I can look to solve this problem. Thanks.

[Niti QX4]

Have you checked the blend door? Listen to see if it moves when you turn the knob.

[Chuckl]

I haven't checked that. That's what I was trying to find out what all happened when you turned the temp knob. I will check that. Thanks.

[VStar650CL]

The way all modern Nissan HVAC systems are set up, the heater core has constant coolant flow and there's a "mix door" which takes air flow through either the core or the A/C Evap or both according to your temperature selection. Incidentally, the constant coolant flow is the reason clogged heater cores are very rare on later models, where it was very common up through the 80's where Heater Control Valves were still the norm. You can find out all about your HVAC control system here...
https://www.nicoclub.com/service-manual ... %2Fhac.pdf
...and your ducting and ventilation here...
https://www.nicoclub.com/service-manual ... %2Fvtl.pdf
...and the basic HVAC system construction here:
https://www.nicoclub.com/service-manual ... 5%2Fha.pdf

[Chuckl]

I listened really close and cannot here any door moving. I haven't had a chance to look at the manuals posted yet. I will have to wait on that until tonight. I guess it would either be the motor driving the door or the module itself that tells the motor to move. Would there be a fuse that could be involved? Thanks.

[VStar650CL]

No fuse, if that was blown then the whole HVAC would be down. The control units almost never fail, but the actuators are garden variety servomotors with balance resistors for positioning. They're basically "toy boat" motors with gazillion:1 nylon gearing, so they do wear out. That's the only good news. The bad news is, the mix actuator is located on top of the HVAC Unit. So if it is a bad actuator, the dashpad has to come off to replace it.

[Niti QX4]

If you are not hearing the blend doors move, then the motor is most likely bad. Sounds like a lot of work, but YouTube is your friend.

[Chuckl]

Printed the manual and have been looking at it but it will be this weekend before I have a chance to look at anything on the vehicle. It would be nice if it wasn't that motor and something more simple. I also wonder, because it would come and go, if the door isn't getting stuck. That would be just as bad as the motor though. I have decided I am not taking that dash apart though. Might be expensive but I will let that be someones else's problem.

[Niti QX4]

Understood. It's not hard but it's time intensive.

[VStar650CL]

That would be just as bad as the motor though. I have decided I am not taking that dash apart though. Might be expensive but I will let that be someones else's problem.

That would actually be much worse. To replace the whole HVAC, not only the dashpad but also the steering crossmember needs to come out, plus both the coolant and A/C need to be drained and then refilled. One is a 4~5 hour job with a $45 part, the other 8~10 hours with an $1150 part.

[Chuckl]

This weekend is going to be the first chance I have to look at this. The manual says to check continuity and voltage at some points on the harness. It doesn't exactly say where this harness connectors are at. The connectors are labeled M50 and M147. Am I to assume they are located on the motor itself which is behind the dash or can I get to it from the air mode panel? I also tried to do the self diagnosis function for the front air control, but could not get it to enter the diag mode. I was hoping that may tell me something. If I could have done that the manual says it would have told me if the air mix door motor circuit was malfunctioning. Does anyone know about this self diagnosing? Seems pretty straight forward, but I could not get it to enter that mode. Thanks.

[VStar650CL]

To find connector locations you need the PG (Power Supply, Ground and Circuit Elements) section of the FSM:
https://www.nicoclub.com/service-manual ... 5%2Fpg.pdf
The "M" designation on the connectors means they're both in the Main Harness that runs underneath the dashpad. "E" is Engine Room Harness, "B" is Body, "D" is Door, "R" is Roof, etc. The only one that doesn't follow the pattern is the engine control EGI Harness, which is "F". You'll find the M harness for your ride starting on PG-43.

For pinout diagrams, they're always found on the pages immediately following the wiring diagram, which in this case is in the HAC section on HAC-45. The pinouts give the associated wire colors, and unless noted are always displayed "harness side" (the side where the wires enter). This is shown by the "H.S." in the box beneath the connector ID, the rare connectors which are shown "terminal side" will have a "T.S." in the box.

[VStar650CL]

PS - Since I don't think you'll be able to access M147 without pulling the dashpad, your best bet to check the Mix Door function is to back-probe M50 and see if the proper voltages are there when you operate the Mix Door. Test 2 in the diagnostics will tell you whether the FAC is sending proper power signals to the door motor, and M50-6 (Sky Blue) should have a voltage that moves from near 0V to near 5V as you adjust the temperature. If Test 2 fails or Sky Blue passes then the FAC has an issue, otherwise the problem is in the door motor or wiring.

[Chuckl]

Thanks I will try that. Have you ever tried the self diagnosis function on page HAC-16? Seems simple but I can't get it to enter self diagnostic mode.

[VStar650CL]

Some of them are very fussy about key timing and sequence, and the manual isn't always explicit about it. Some can take many tries to initiate until you figure out exactly what they want. Keep trying, that's all I can suggest.