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How to Clean Evaporator Drain Tubes

dashboard showing AC light

Modern AC systems are comprised of multiple individual components that convert warm air inside the cabin to cool and refreshing air. However, there are times when the air that blows into the cabin is not as refreshing or as cool as it should. Although there are several reasons that lead to poor air conditioning performance, one of the most overlooked are problems with clogged or dirty evaporator coils or obstructions inside the evaporator drain tube.

When water is contained within any item, the introduction of heat and oxygen allows microscopic organisms that live in our water a prime environment for growing into mold and harmful bacteria. This bacteria attaches to the inner metallic parts inside the evaporator and can restrict the flow of refrigerant and liquids inside the device. When this happens, pieces of bacteria or debris break off the coils and can become trapped inside the evaporator drain tube, since in most cases it has a 90 degree bend. If this happens to you, the evaporator drain tube needs to be cleaned as well as the evaporator itself.

The AC drain hose or evaporator drain hose, as it's often referred to, is located on the engine compartment side of the firewall. On most domestic and foreign vehicles, the AC evaporator is inside the cabin, directly in between the firewall and the lower portion of the dashboard. Most car owners and DIY mechanics choose to clean the AC drain hose if symptoms show up (which we will highlight in the next section below) as opposed to removing the evaporator case and completing an intensive cleaning of the evaporator.

It is recommended by ASE certified mechanics as well as automotive manufacturers that you clean the evaporator case from the vehicle and clean that unit at the same time as you are cleaning the evaporator drain hose. The reason you want to complete this extra step is due to the fact that the debris causing the AC drain hose to not function correctly is inside the evaporator case. By simply cleaning the tube, the problem will come back sooner than you'd think and this process must be done over again.

We will show you the steps that you should follow for cleaning the evaporator case and cleaning the internal components of this critical AC system as well as removing debris from the evaporator drain hose.

Part 1 of 2: Determining the symptoms of a dirty evaporator drain tube

Dirty evaporators display multiple symptoms that indicate that it is dirty and needs to be cleaned. The evaporator is designed to convert warm and often humid air into dry and cooler air. This process removes the heat and humidity by using refrigerant circulating through a series of metallic coils. When this occurs, the humidity is converted to liquid (H2O) and needs to be removed from the evaporator in order to reduce the build-up of mildew and mold. Noted below are a few of the common warning signs that a problem with the AC evaporator exists and that it needs to be cleaned.

Musty or dirty air coming from the AC vents: When the inside of the evaporator collects bacteria, mold and mildew, residue seeps into the air that it is trying to cool. Once that cool air is circulated through the vents, it is contaminated with bacteria that often cause a musty or dirty smell inside the cabin. For most, this musty and dirty air is more of an annoyance; however, for those people who live with Chronic Obstructive Pulmonary Disease or COPD – which includes 25 million people in the United States according the CDC - the bacteria in the air can trigger exasperations or COPD flare-ups, that often cause a hospital visit.

The air conditioning system is not blowing air consistently: Another common symptom that alerts a vehicle owner to a problem with their evaporator is when the air blowing into the cabin is choppy and not smooth. The AC system has a system of controls that powers the fans to operate at a desired speed. When the inside of the evaporator is clogged with debris, it causes an inconsistent flow of the air into the vents.

The inside of the vehicle is developing a bad smell: Since the evaporator is located between the dashboard and the firewall, it can develop a bad smell when it's clogged with excess bacteria and debris. This find its way into the cabin of the vehicle eventually, creating a really bad musty smell.

When the bacteria and debris form inside the evaporator, it breaks off and flows into the evaporator tube. Since the tube is commonly made from rubber, and typically has a 90 degree elbow, the debris blocks the inside of the tube, which reduces the flow of condensation from the evaporator. If unrepaired, this causes the evaporator to break, potentially leading to expensive replacement or repair. In order to reduce this possibility, cleaning the evaporator and removing the blockage in the tube through the steps we'll outline below is typically the best course of action.

Part 2 of 2: Cleaning the evaporator drain tube

On most domestic and import cars, trucks, and SUVs, the AC system follows a similar pattern to the diagram above. The evaporator is typically located on the passenger side of the vehicle and is installed between the dashboard and the firewall. In order to clean it, you don't need to remove it. In fact, there are several OEM and aftermarket AC evaporator cleaner kits that include one or two different aerosol cleaners sprayed into the evaporator when attached to the evaporator tube.

Materials Needed

To complete this task, you'll need to make sure you have easy access to the evaporator drain tube. On most cars, trucks and SUV's, this tube will be located in the center of the vehicle, and in many cases, next to the catalytic converter. Make sure you prepare your vehicle for service by raising it on a hydraulic lift, or raising the vehicle on jack stands as indicated in the sections above. You won't have to remove the battery cables as you won't be working with anything electric during this cleaning service.

Step 1: Raise the vehicle. Ensure you have clear access to the undercarriage of the vehicle.

The problem with using jack stands is that sometimes the fluid is trapped inside the evaporator and does not drain fully from the vehicle as it is elevated. To combat this, raise the entire vehicle on four jack stands.

Step 2: Crawl underneath and locate the evaporator drain tube. Once the vehicle is raised enough for you to have clear access, find the evaporator drain tube.

On many cars, trucks and SUV's, it is located very near the catalytic converter. After you've located the tube, place the drain pan directly underneath and make sure you have the can of evaporator cleaner available for the next step in this process.

Step 3: Secure the nozzle from the can of cleaner to the bottom of the tube. The can of cleaner typically comes with a secondary nozzle and spray tube that inserts into the evaporator tube.

To complete this step, follow the instructions from the manufacturer of the evaporator cleaner. However, in general, you should remove the top of the can, secure the nozzle tip into the evaporator drain tube, and apply the spray trigger onto the can.

Once you've attached the spray nozzle to the can, in most cases, the can automatically begins to stream foam cleaner into the evaporator. If this doesn't occur, proceed to the next step.

Step 4: Empty ½ the contents of the can into the evaporator. In most cases, the can automatically streams the cleaning agent into the evaporator.

If this doesn't occur, simply press down on the spray nozzle on top of the can to insert the cleaning foam into the evaporator. The directions on most products recommend spraying ½ of the can's contents into the evaporator, letting the foam soak for 5 to 10 minutes.

Do not remove the nozzle from the evaporator drain tube or the contents will spill prematurely. Wait for a minimum of 5 minutes before removing the tube.

Step 5: Remove the nozzle and let the contents drain. Once you've let the foam cleaning agent soak in for a minimum of 5 minutes, remove the nozzle fitting from the evaporator drain tube.

After this is done, the liquid will begin to flow from the evaporator quickly. Let the contents inside completely drain from the evaporator.

  • Note: While the evaporator cleaning agent is draining, you can save some time by preparing the next step of the cleaning process. You'll need to remove the cabin filter inside the vehicle. Many mechanics let the fluid drain until it's a slow drip. Leave the pan under the vehicle, but lower the vehicle from the jack stands or the hydraulic lift. This expedites the flow of fluid inside the evaporator.

Step 6: Remove cabin filter. Since you're cleaning the evaporator and the evaporator drain tube, you'll also want to remove and replace your indoor cabin filter.

Follow the instructions for completing this step in your service manual, as they are unique for each vehicle. If you intend to use the cabin filter cleaning agent that is included in most evaporator cleaner kits, remove the filter and insert the cartridge before completing the steps noted below. You do not want to have the new or old filter in the cabin cartridge as you're spraying the cleaner into the vents.

Step 7: Clean the AC vents. In most evaporator cleaner kits, there is an aerosol can that is designed to clean the inside of the vents.

This improves the smell inside the vehicle and removes potentially harmful bacteria trapped inside the vents. The general steps for doing this are, first, removing the cabin filter and starting the car.

Turn the AC off, open the vents to outside air, and turn the vents to the maximum setting.Roll up windows and spray all contents of the aerosol cleaner into the window vents located below the windshield.

Turn the vents off and turn off the vehicle.

Step 8: Keep the windows rolled up for 5 minutes. You’ll then roll down windows and let the vehicle ventilate for 30 minutes.

Step 9: Remove drain pan from underneath vehicle.

Step 10: Lower the vehicle.

Step 11: Clean the interior coils. Once you've completed this process, the evaporator drain hose should be unplugged and the interior coils in the evaporator cleaned.

The cleaning agents are designed to continue scrubbing the coils for a while until the condensation naturally expels them from the vehicle. Sometimes you may find a few stains on the driveway for the first few weeks after completing this process, but those stains typically wash away rather easily.

As you can see from the above steps, cleaning the evaporator drain hose is one of the easiest jobs to complete. If you've read these instructions, reviewed the service manual, and have determined that you'd rather have a professional complete this service for you, have one of YourMechanic’s certified mechanics complete the evaporator drain hose cleaning for you.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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