When outside temp is around 90F, after AC is turned on, it feels cold initially. But after 1 hour or so, it only blows cool but not cold air. This is the case for both auto on and off with both side temp control turn down to LO.
The inside of the car feels a bit too warm but not too hot.
After I turn off the engine for 20 min and restart, It repeats that that cold to cool air blowing again. Any idea what could the issue? Thanks.

 

Sounds like some freeze up is occurring - then you shut down, it melts and normal operation returns. Not sure what the fix is. My daughter had this with her Q5 and a part had to be replaced.

 

How old is that car? It may be that it needs the refrigerant to be topped-up.

 

Does the car make a jet sound in front of the passenger seat when accelerating while the ac is acting up? Freezing usually means that there is too much freon, when the evaporator coil freezes the pressure on the HIGH side will be excessive which will cause excessive noise around the blower motor.

 

Any abnormal noise when the issue happens. I am thinking that the device controlling temp may stuck at say 77F (even I set it to LO). Just a thought.

 

Just some update: I drove the car for a trip for couple of hours. This time, the outside temp was around 95 and i did not find any issue. The ac always below cold air. After I came home, I did some measurement and the outlet temp was around 40F and there is quite bit condensing water coming out from the bottom.
I think there is no issue with compressor or refrigerant. I also tested the temp control and it works for both driver and passenger side. I am thinking that the issue might have something to do with outside temp and humidity so that the evaporator could be frozen and prevented air going through (as suggested by mikemu30). I have previously change the cabin filter with a FRAM filter. Maybe it is different than the OEM one. I think I might get OEM one and change it back.

The car is still under warranty. But without a way to show the issue, I think the dealer can not do much. Anyone has any thought, please let me know. Thanks.

 

Could the drain be blocked?
If you do end up at the dealer change that cabin filter back to OEM or else you may feel like a ping-pong ball

 

"and there is quite bit condensing water coming out from the bottom."

 

Missed that.

 

I have used FRAM cabin filters for the last 4 years on mine. No issues with the AC in Texas with high humidity and lots of heat (that gets hotter and drier each year)...

 

Today, I connected my OBD II WIFI scanner and checked "Evaporator Fin Thermistor" reading. It appears working and stayed at 32F after 10-15 min of running. It is supposed to control the cycle of AC so that the fin will not get frozen. But I think 32F is a bit on the boundary of getting frozen. I am thinking a slight change such as the cabin filter may change the equation. Thanks.

 

It happened again two days ago when driving in high way after around 1 hour. This time I got enough evidences toward evaporator iced up problem:
1. When it happens, the out let air flow reduced noticeably.
2. After turning off the car for 15-20 min, the issue is gone, but will come back after another 1 hour or so.
3. Keep fan high blowing speed will avoid the issue (but will not resolve the issue if the problem already occurs).
4. After stopping the car, it seems to drain a lot of condensation water (more than a normal case).

I've sent the car to the dealer. Not sure what can cause that. Thanks,

 

Generally if ice is present it's low on A/C fluid/gas

 

Thanks, but I don't understand the reason. So, low on refrigerant causing evaporator go colder? Is that what we want?

 

Yes. It seems counterintuitive, but diminished refrigerant causes the system to work overtime and it eventually can freeze up the coils.

See No. 4 in this link: https://trustcompleteair.com/frozen-...r-coil-causes/