Hopefully it is just a refrigerant issue, but I recently had similar symptoms where the A/C would blow warm air intermittently and it was diagnosed by the dealer as a compressor failure.

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Hi Dan... Welcome to the Forester forum...

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Next up, the AC system issues are documented and have been experienced by many. There are known issues with the AC Compressor (some years) and the AC condenser (other years) in the SJ generation Forester (2014 - 2018). Use the SEARCH COMMUNITY feature of the forum and you'll see a few threads and discussions.

Chances are you either have a bad condenser or you're low on Freon. But until you have a thorough examination of the AC system, any responses here will just be conjecture based upon their experience and may - or may not - be what you're having the issue with.

I will note, however, that there are times when the Forester's AC system just can't quite cut it. It's just a weak point in extreme situations.

 

Try checking the clutch gap, if it's over 0.6 mm then it's too much and will need a spacer removed. Check out this video, helped me a lot to diy it:

 

My wife complained all year that her 2017 foerster a/c would work and then not work. After several months driving the vehicle here is what I found and a solution. The dealer wanted to replace the compressor at $1400 and I told him to pound salt. When driving the vehicle and the A/C quit working, if I turn the temperature all the way to “Lo” it would work fine at high fan speed. Obviously this “Lo” setting is bypassing something. Any attempt to adjust the temp or fan speed the AC clutch would turn off. I also noticed that if I was driving above 45-60mph the a/c would shut off until I turned it to ”Lo”. Most of the time it worked at slow driving speeds but sometimes it wouldn’t. Then I noticed if I turned the headlights on, the A/C worked fine all day no matter what speed I was driving. At first I thought this might be a voltage regulator issue, but using a DVM on the battery showed no difference at 2200 rpm with the lights on or off. I am suspicious of the cabin temperature sensor. I believe it is an RTD and could be susceptible to noise or a poor ground. This is the only sensor that could reasonably be bypassed when switching to “Lo”. No engineer would bypass the high pressure switch, or low pressure switch or the evaporator temperature sensor. So my best guess is something to do with this sensor.

For now, I told my wife to drive with the headlights on, and she is happy as a clam. If I dig a little more and find something, I’ll post my results.

 

@JPG... This is exactly what happened with me. While being puzzled about why the AC would work perfectly well when it's cool outside and I don't need it but on a long hot summer road trip it was pumping out warm air, I noticed that there would be a sudden burst of cool air whenever I changed lanes! After contemplating whether I should weave like a drunk driver to my destination, I realized it was by turn signal blinkers that were doing the trick. I then tried just turning on the parking lights and voila! I hade cool air for the entire trip.

I think this is an electrical issue, perhaps a problem with the AC relay switch but it's obviously not the compressor. Have you figured out a permanent fix yet?

 

Remember also that in some cases, by keeping the temp in "LO" and especially if in RECIRC mode, you run the risk of over-cooling the freon and this can cause the system to stop the "flow" of Freon through the system in order to raise the temp on the freon and keep the system from "freezing".

LO is not a temp. So in LO the system will just keep cooling and cooling and cooling. It's better to set a low temp and let the system work as well.

Also, check to see if you are full on freon and you're not low; this can also case the system to shut off intermittently to keep from being over-worked and possibly over heating the parts.

In your owner's guide, you'll find some different operation 'notes' about the AC system and how it works. When in doubt, read the manual.

Operating tips for heater and air conditioner ...................................................... 4-11​

Cleaning ventilation grille................................... 4-11​

Efficient cooling after parking in direct sunlight ........................................................... 4-11​

Lubrication oil circulation in the refrigerant circuit.............................................................. 4-12​

Checking air conditioning system before summer season ............................................................ 4-12​

Cooling and dehumidifying in high humidity and low temperature weather conditions ................. 4-12​

Air conditioner compressor shut-off when engine is heavily loaded.............................................. 4-12​

Refrigerant for your climate control system ........ 4-12​

Finally - even the best HVAC system in the world can have difficulties keeping up with exceptionally hot days and higher humidity.

 

Thanks for your notes. A couple things. My freon and pressure was checked by a Subaru dealer mechanic and they said it was fine. They did say the compressor was bad, but I already knew it was not. So I paid the $100.00 inspection fee and went on my way.
I did clean the ventilation grille. I also checked the gap on the A/C clutch and it was fine. Now as automation engineer I know that if I was writing code for this, there will be a few things to note. When in “Lo” I would never bypass the low pressure sensor or the high pressure sensor, since bypassing those could very well damage the compressor. I would also not bypass the evaporator freeze sensor, since that could cause problems with air flow and damage the evaporator. The only thing I would bypass in software is the “in cabin RTD temperature sensor. Because when a driver gets in a hot parked car, he wants it to cool down fast.

Since I don’t have an electrical schematic, I don’t know if there are any other sensors In the A/C system.

I don’t know what the definition of “heavily loaded” is but as a driver, I don’t care about engine efficiency or mpg, I just want the A/C to work when I command it.

Maybe you can pass this on to the Subaru engineers.

 

I'm not a part of Subaru - just an owner like you.

Sadly, with an emphasis on MPGs (not only by Subaru but across the market), things like this will become normal.

I live in another hot climate - So Cal deserts - and so I know the ".. when a driver gets in a hot car, he wants it to cool down fast..."

When I bought my Forester in 2013 (a 2014 Forester), it was late July. I drove an XT (beautiful Venetian Red Pearl) with the black interior. Over the test-drive loop, the front seats were just feeling "cool" and the back seat (sales guy was there) was still very warm. The loop is about 2 miles and drive was maybe 15 minutes? Test drive #2 was the Ice Blue metallic with the "platinum" leather interior. By the end of the same test drive loop, the front seats were very cool and the back seat was cooled down.

I'll agree that the AC system is not the best I've ever had. My old Chrysler felt downright frigid when front and rear ACs were blowing at max (Town & Country minivan). My old Mazda (which was traded for the Subaru) was also pretty bad at cooling the interior down quickly after a few hours in the sun.