I have a 2011 Pathfinder. Base model. Auto trans. 45k miles. Stock. No problems untill this:

When the vehicle is warmed-up, and running, the heat stops working shortly after starting the trip. Air flows from all vents as its supposed to but it begins to blow cold air after only a few miles. No vents are blocked, and coolant level, and thermostat are okay. Recirculate feature works. Today we started it in the driveway and as soon as the engine started to warm, heat was produced from all settings selected. Then when the engine reached operating temprature, only cold air came out no matter what settings were selected. Wife reports this happens also when stopped for traffic lights, ETC. It has been eceptionally cold here in the Southeast lately, but I am perplexed as to what could be causing this problem. Any Ideas, anyone? p.s. Warranty has expired

Burping the coolant system would be the first thing to do. Lots of posts around here on that. Apparently these are known for getting air pockets in the coolant.

That's happened to me a few times. I'm not sure exactly how it happened, but I remembered having the A/C on with the heat, the truck was warmed up and turning the A/C off seemed to mess it up.
So now I just make sure that the A/C is off when I put the heat on unless it's raining.

I'm literally fighting with this as we speak. Just as the car heats up, I get polar vortex quality air from all vents. Will try to burp when it warms up a little. Thanks!

skinny2 wrote:Burping the coolant system would be the first thing to do. Lots of posts around here on that. Apparently these are known for getting air pockets in the coolant.

I tried this, but just parked it on an uphill slope without Jackstands, and IT WORKED! Thanks for the help, Skinny2!

You are dealing with a known, common flaw. You have a leak in the radiator supply line that feeds the rear heater core. This leak is caused by a change in radiator fluid pH that corrodes the thin aluminum line from the inside. The BRILLIANT engineers at Nissan ran this thin, aluminum radiator supply line BETWEEN the frame and the body. This line normally starts to leak at the rear of the vehicle behind the passenger rear tire. The only cure/fix of this problem is to lift the body off the frame and replace aluminum supply line (cost prohibitive). You can do what I did to stop the leak. I forced a rubber inter tube patch repair piece between the frame rail and body and wrapped it around this aluminum supply line. I then zip tied the rubber tightly around the supply line to stop it from leaking and preventing air getting in the system causing air locks that in return cause the rear heater to blow cold air. This is a similar problem with the thin aluminum transmission cooler that can corrode over time allowing radiator fluid to destroy the vehicle's transmission. I hope this helps.

Actually, the trans cooler in the radiator doesn't corrode; there is a seal that fails and causes the cross-contamination issue. FYI, as far as the rear coolant pipe, the pipe can be repaired using a an A/C repair kit, like Dorman #800-600, rather than slipping on heater hoses or lifting the body off of the frame and replacing the entire line set, depending on where the leak is. They usually don't corrode on the inside of the pipe, but on the outside. Dirt tends to get in under the rubber insulating pads around the pipes at the brackets, where it scratches its way through the pipe finish.

If this aluminum line is indeed leaking, would there be enough leakage to drip to the ground? This Pathfinder has needed no coolant added that I can remember in almost 60k miles. I recently changed the oil and filter and noticed no leakage anywhere under the front, but I will keep my eye on this!
Thanks again for your help!

Depends on the size of the leak, but it would be fairly easy to spot the green anti-freeze by taking a look under the vehicle. There are several A/C and coolant lines that runs together near the top of the right side frame rail to the heating and cooling unit at the right side, rear of the vehicle. Air pockets in the coolant system affecting heat are far more common than leaking lines.