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Forester AC Problem

For at least a year now, my 2004 Subaru Forester has had some kind of AC problem that generally fits a pattern: it doesn’t like to work when it’s hot out - 80 or so, and if it’s borderline hot low 80’s vs. say - 95), and I stop and then go again (for say, turning, which of course involves stopping, turning and accelerating), it starts up again for awhile. The temp guage in the car appears to be operating but not overheating. I run the AC all winter in humid WI/MN winter conditions, and it always works (of course). My mechanic noticed slightly low coolent, but said it wouldn’t be low enough to explain the problem. Plus, then he filled it and ran it a couple times, and had the same problem when it got hot, even though he said it appeared to be fully charged. At 140,000, it runs like a charm aside from this annoying thing. I’ve not taken it to the dealership, and would like to avoid it. Help: we’re driving through South Dakota in early August!

The AC system in your vehicle has an engine coolant temp sensor. This is there in the event the engine should start overheating, the AC will automatically shut off to prevent any added heat from the condensor from being applied to the radiator. Thereby possibly preventing the engine from overheating.

You might want to have this coolant temp sensor checked to see if it’s operating properly.


Thanks Tester. I’ll put it on the list.

I was reading that if the ECT is malfunctioning, I’d have problems with either low fuel economy (which I don’t), or cold driveability problems, including cold starting (which I don’t either). Ya never know though, and it sounds like a relatively cheap fix.

There may be more than one coolant temp sensor. Tester said “The AC system . . . has an engine coolant temp sensor.” I take this to mean the AC system has its own sensor, separate from the sensor that feeds information to the engine computer and/or the temperature gauge in the dashboard.

It could also be your engine is getting on the verge of overheating due to a clogged radiator, sticky thermostat etc. Does the temp gauge go to the high part of normal when the AC quits?

No, the temp guage moves from 0 to no more than the middle as the car heats up, and stays there, slowly lowering back to 0 once the car’s turned off. In 2006, it did overheat, because no one had checked the condition of the radiator cap gasket, and collant had leaked out. I did just have a flush/fill. That would eliminate that, right?

Sounds like there’s no actual overheating.

Maybe not engine overheating, but the AC still doesn’t work when you actually need it to work.

Could it still be the AC engine coolant temp sensor is somehow doing this?

AHA! It did overheat! You know what takes out coolant temp sensors the most? Overheating! This one overheating event may have damaged the coolant temp sensor for the AC system.


My text indicates “ECT sensor problems are most often caused by wiring faults or loose or corroded connections rather than failure of the sensor itself”.

Clogged cabin air filter. Has it ever been changed?

Blocked/dirty radiator between A/C condenser coil and radiator?

If equipped, a evaporator temperature thermistor not reading properly

Interesting, about the coolent temp sensors, Tester. Although when I think about it, the AC hasn’t been malfunctioning since the overheat. It was at least a yr later…

oldschool, I bought this car new, but it had been on a dealer lot where it was exposed to acid rain. Early on, there were some gaskets that I had to replace that my mechanic said were “older” than they should have been, given when the car was made.

All good thoughts americar, and they’ll go on the list.

I like to go forearmed to the shop, especially on something like this, which has (by what I’ve read online) challenged all kinds of mechanics.

Thanks, people!


Did you ever resolve this? If so, please post the fix. I’m having the exact same problem. Thanks.

I haven’t. We went into a tailspin after mom-in-law’s cancer diagnosis. I hope to before the end of Sept., cause it’s starting to misbehave when the ambient temp is in the 70’s. Winter driving could be very - foggy. I also posted this to, (same username) and got some interesting answers. Good luck to us both!

BTW, my husband corrected my memory on the overheating - it happened on my last car, not this one. I rack up the miles, about 35,000/yr, so my vehicles and their issues and events all kind of blend together.