Air circulation turns off by itself and stinks

[Buggs1a]

Hi. When I am using my heater/ac and using defrost vents or anything not straight out I have it set for inside air only. It turns this off by itself then there’s a smell. I can tell immediately because the fan speed changes and the smell. I’ll push the button back to inside air. After a bit it’ll go back to outside air by itself. I use the windshield and feet setting. The only time it stays inside air is when using the front vents that hit you in the face. On the display it’s the arrow pointing right. That’s the only time the setting I chose for inside or outside air will stay put. But even then it has switched on it’s own to outside air sometimes. Just not as often. And the bad smell is not an outside smell. I have already had the Ford dealer change a filter but that didn’t help. Temp settings don’t make a difference. 
This is on my 2013 cmax sel. 

[cr08]

I have found, and I believe this is mentioned to some degree in the owners manual, that the HVAC system does have programming to monitor interior humidity and will adjust/turn off recirc if it feels it needs to to maintain a certain humidity level. I had noticed the same behavior on mine (not necessarily the smell, but the recirc mode turning itself off) and had to dig around on that.

[Buggs1a]

Is there any way to stop this? 
why does it do this? It thinks it’s too humid or something? Weird. 
thanks. 

[cr08]

No way that I'm aware of to stop it. But you are correct, it does it if it deems the air in the cabin is too humid. The verbiage on this is in Ford's workshop manuals:

 

In-Vehicle Temperature And Humidity Sensor

The in-vehicle temperature and humidity sensor contains a thermistor and a sensing element which separately measures the in-vehicle air temperature and the humidity, then sends those readings to the HVAC control module to adjust the actuator doors. The in-vehicle temperature and humidity sensor has an electric fan within the sensor that draws in-vehicle air across the two sensing elements. If the vehicle cabin becomes too humid the HVAC control module adjusts the air inlet door to allow more fresh air. When the humidity level drops, it adjusts back to recirculated air.

 

That said it doesn't specifically mention the recirc icon turning off but I'm assuming it is related to all of this. In my personal experience I've noticed it happening after running with recirc for a bit and the windows just begin to start fogging up.

[Buggs1a]

So the smell. It’s horrible. It happens when the defrost is on. 
So I’ve got the air set for inside. The smell is fine. I have the windshield defrost on. And feet on and or the dash on. The air goes automatically to outside air then smells horrible. I can tell immediately when it changes because the air pressure is less. And it smells horrible. I have to turn on inside air again. Then the bad smell goes away and air pressure ramps up a bit. This happens all the time. 
 

If I use only the dash front vents this don’t happen. The air stays on inside. No bad smell. Only when defrost is on with feet or center on does it automatically shut off inside air and go to outside air then smells bad. This smell is not outside smell. Because outside is fine. 
 

im desperately needing to fix this smell issue. I took car in before and they changed air filter. No idea which filter. Smell maybe was ok for a bit. Then crummy smell again.