I have a friend who has a 2015 Lariat FX4 with about 37k miles on it. When he is driving down the road the climate control works normally. When he stops and the vehicle is idling, it begins to blow cold instead of warm air. I guessed an actuator door maybe, but don't know. My wife had one malfunction on her Mustang that caused a couple of vents to blow hot instead of cold air in the summer, but it did it all the time, not off and on like his truck. Nothing has been replaced on the truck and no previous issues with the heater core or anything. Any thoughts on what could be causing this issue? Thanks, as always for your insight.

 

Low coolant and/or air trapped in the block.

 

My ranger does this when it's low on antifreeze. When there isn't enough to cool the block the heater core only gets antifreeze every other time the thermostat opens.

 

He just messaged me that his truck was about a half gallon low on coolant, so I bet this fixes the issue. I'll report back. He's out testing it now...

 

Check the heater hoses and make sure they are not collapsing. At idle, the water pump puts out a fraction of the pressure it does when its at speed.

It could also be a partially plugged heater core. There were two TSBs related to plugged heater cores on 2015 and 2016s.

 

Here is the other one.

 

I bet that's it. In the summer it won't run hot but when you send it to the heater core it drops to low.

 

I think the crisis has been averted. He said after adding coolant it appears to be working normally. Thanks for the responses everyone. Still the best source of info on the web....

 

He should now find the coolant leak. I know my truck just turned a 100k and I did not have to add any coolant at all over the last 3 years.
So I think he could have an issue.

look at the hose fitting, water pump and maybe even the oil cap.