(Based on previous questions)

Background: I've been having trouble with the AC in my '04 Corolla. The compressor was replaced and subsequently the shop reported bad fuse contacts which they addressed. However, the AC didn't cool in a significant, consistent way even after the fuse issue was addressed.

My next step was to get a second opinion from another mechanic, I'll call him Mechanic #2. He asked me to open my glove box and pull out the AC filter to look at it. It was filthy. Since I will need to replace it anyway, I didn't bother putting it back in.

I feel as though the system has been performing markedly better since I pulled out the filter. Is this just my imagination? Or can pulling a dirty filter out make a significant difference in the cooling effects of the AC system?

Also -- is the filter something I can pick up at an auto parts store?

(Note, I'm not asking whether this is the whole solution to the problem -- obviously there's no way anyone could answer that!)

  • " is the filter something I can pick up at an auto parts store?" Yes
    – Moab
    Commented Jun 11, 2018 at 15:47
  • Clarification: the intention was never to leave the filter out long term. I called the parts store and they ordered the filter for tomorrow. Also note, it wasn't hot here today. // I wanted to understand whether the increased performance I thought I was seeing after removing the filter might be due to the removal of the filter. Commented Jun 12, 2018 at 3:46
  • Update: I put the new filter in today and tried out the AC. Basically, it sort of works, but it's wimpy. Not sure where to go from here.... Commented Jun 14, 2018 at 1:04

3 Answers 3


Yes, but it is not the cooling capacity of the A/C system that has changed, it is now that the air coming into the cabin can actually move through the filter, well filter box since you have not replaced it, and the system can now do its job of cooling the air.

As for effect, it means that any particles, flies, bugs etc can now get trapped in the evaporator, which if it gets so bad it needs cleaning, is a very expensive job as most of the dash of the car has to be removed in most cases.

Far better to fit a new filter...

Edit re comment: the cooling capacity I am referring to is the designed capacity between the evaporator and condenser which will not change whether the filter is dirty or not. Having calculated and plotted on H-S diagrams many systems the effects of allowing the evaporator to be clogged by dirt, flies etc reduces the effectiveness markedly.

  • Wrong. With a dirty cabin filter, the real cooling capacity was much lower. Yes, he'd better fit a new cabin air filter as soon as possible, otherwise the cooling capacity will, again, decrease, this time due to a reduced evaporator's heat exchange surface. He doesn't have to fit an activated charcoal one, though. A normal one is enough if money is a problem.
    – Al_
    Commented Jun 11, 2018 at 18:11
  • 1
    @Solar Mike you are correct .The A/C cooling capacity doesn't change 12000 Btu is still 12000 Btu . What a clogged cabin filter does is restrict the transfer of energy from the evaporator core to the air. If it is clogged too much the evaporator will freeze up and become a block of ice and stop working at all until the situation is corrected,
    – Old_Fossil
    Commented Jun 13, 2018 at 4:12
  • @resident_heretic Any AC system has a control mechanism (such as an evaporator temperature probe, a compressor stroke pneumatic control valve/solenoid, a low pressure switch or a POA valve) that, when working properly, prevents the evaporator from freezing up by either reducing the refrigerant flow or turning the compressor clutch coil off.
    – Al_
    Commented Jun 13, 2018 at 7:55

The cabin filter is a relatively new acssesory to improve the comfort of the passangers by cleaning the air. Unless dirty, it has no impact on the system at all. Cars have been driven around for decades without any filter or extra maintenance at all right here in dusty Phoenix Arizona. The filter is an optional accessory based on trim level for many cars. For example, on some cars the such as my V6 Challenger you can buy the filter box found on the higher priced version and easily swap it with the non-filter piece.

  • Problem is, when there's no cabin air filter, you have to be extra careful about keeping the evaporator clean...
    – Al_
    Commented Jun 13, 2018 at 7:57
  • @Al_ exactly the point I made in my answer - see paragraph 2 about flies bugs etc getting trapped in the evaporator...
    – Solar Mike
    Commented Jun 13, 2018 at 8:01
  • Nobody cleans evaporators of the older systems and they work just fine. There was no such thing as a cabin filter until the early 2000's. No cleaning needed. Same as not having to clean the condensor. Commented Jun 13, 2018 at 14:02
  • The problem is, modern AC systems are designed to work on the lowest refrigerant charge possible, unlike the old R-12 ones. They're, therefore, designed with efficiency in mind (i.e. parallel flow evaporators and condensers vs serpentine ones, TXVs vs OTs, variable displacement compressors vs cycling clutch ones and so on) and any decrease in heat rejection or absorption capacity can have a more noticeable impact on performance.
    – Al_
    Commented Jun 13, 2018 at 15:26
  • The existance or non-existance of a cabin air filter being dependant only the trim level is evidence that there is no difference. The manufacturers are not putting more refrigerant in the base models and less in the premium versions. Commented Jun 13, 2018 at 15:37

Cooling capacity of the interior is affected by the cabin air filter's resistance. In fact, the equation to cooling capacity includes the evaporator airflow's mass rate. It's true that with a lower airflow the air can be cooled better by staying a longer time between its fins, but if the vents can output a larger amount of air, then the hot/warm air of the interior is replaced by the cool air outputted by the ac system in a faster fashion so effects from any heat source (such as solar radiance) can be countered better (and the result is a cooler temperature), even though that air is actually warmer.

If you look carefully, you'll find that any vehicle has vent ports that allow air to exit the interior and prevent any pressure gradient from forming between the vehicle's interior and the ambient. That's why i'm using the verb "to replace", either with recirculation off or on (recirculation just helps the evaporator do its job, just like the condenser fan does with the condenser).

So, yes, a clean cabin air filter makes a huge difference in cooling capacity, provided the rest of the system is performing efficiently.

  • Could you show me your explanation using an H-S diagram?
    – Solar Mike
    Commented Jun 11, 2018 at 18:54
  • Unfortunately i can't, but try jumping in a hot car with manual AC on a hot and sunny summer day, turning the AC system on and setting the cabin blower at the minimum speed. You won't feel comfortable, because the evaporator would be removing heat from the interior (or ambient, if recirculation is off) air at a very low rate and there's a source of heat (the solar irradiance). Now, if the filter were dirty, the evaporator airflow would still be reduced so, again, heat would be removed from the air at a much slower rate. In a nutshell, the system would not be working at its maximum capacity.
    – Al_
    Commented Jun 11, 2018 at 19:38
  • My car has the A/C on permanently winter or summer...
    – Solar Mike
    Commented Jun 11, 2018 at 19:40

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