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My girlfriend's 1998 Mercury Sable LS has climate control. You set the temperature and it decides whether to heat or cool the interior. Problem is, it is always running the AC. Even if I turn on it up to 90F, I can see the AC compressor kicking in and ice cold air is blowing out.

I've run the EATC self-test and and got code 024, so I tried replacing the blend door actuator, but during testing of the new part, it appeared the system didn't even attempt to move it.

This was her mother's car, and she was a heavy smoker. I noticed that the hose labeled "sensor" is almost completely clogged. Could this have something to do with it? I can't find anything only for "heat sensor" or "thermostat" that doesn't relate to the engine. And all the YouTube videos are about manual heating/cooling, and not the Climate Control system installed.

Lastly, could it be that the "head" unit has gone bad? Her niece had the car for the last month and had no complaints, we got it back last week and noticed the AC was stuck on.

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Does it make a difference what area the air is focused on? I know some cars will force the AC on when you have the window selected. The A/C dries out the air, which makes it absorb moisture better and defog the window faster. Check with it pointed just at the footwell to see if it makes a difference.

Otherwise, my best guess would be the sensor. Usually, these systems use a thermoresistor that changes the resistance across a circuit based on the temperature it's exposed to. If this is behaving badly, it could be supplying an extreme value and the computer will think your car is outrageously hot. If the hose that exposes this sensor to the cabin air is blocked, it could be reading inaccurately as well (although I wouldn't expect it to be reading > 90' when the car is cool).

I had a somewhat similar, albeit opposite problem with my car. The outside temperature sensor was reading unreasonably low, so the A/C would never come in. The car thought the outside temperature was sub-zero, so it would just pump outside air into the cabin without ever turning the A/C on.

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I agree with Ray Dowe that it's most likely the in car temp sensor that is not working... but there is one other thing to check.

That is the air conditioning (A/C) clutch air gap. On the front of your A/C compressor is a spring mounted flat plate. When the system needs A/C, a powerful magnet underneath the spinning pulley turns on and pulls on that spring loaded clutch plate. Magnet on, plate is engaged and it spins the compressor. Magnet off the plate SHOULD remain motionless. There is a precise spacing required for the gap when the magnet is off. If that gap is too small the compressor will run when it shouldn't. I believe the air gap should be around 0.65mm to 1.25mm (note please verify this with your service manual. These gaps are NOT the same on all makes and models. )

Check the gap with feeler gauges when the engine is off. Air gap is adjusted via super thin washers underneath the mounting hub. Note if you do this job yourself Autozone offers a clutch plate holding tool in their free tool loaner program. You will also have to have a torque wrench for this job to tighten the hub nut.

Another possible defect related to clutch air gap is a broken or bent leaf spring in the clutch plate. When you are checking the gap check the springs too

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