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How to Replace a Car Air Conditioning (AC) Low Pressure Hose

AC low pressure hose

The air conditioning (AC) system of today's modern cars, trucks, and SUVs is a closed-loop system, meaning that coolant and refrigerant inside the system does not escape unless there is a leak. Typically, the leaks are found in one of two different locations; the high pressure or AC feed lines or the low pressure or return lines. When the lines are secure and tight, there is no reason why the AC in your vehicle shouldn't continue to blow cool air unless the refrigerant needs to be added. However, occasionally problems occur with the AC low pressure hose which requires replacement, and recharging your AC system.

The low pressure side of the air conditioning system in most vehicles is attached from the AC evaporator to the AC compressor. It's referred to as the low pressure side because during this phase of the cooling process, the refrigerant flowing through the system is a gaseous state. The high pressure side distributes liquid refrigerant through the AC condenser and dryer. Both systems must work together to convert warm air inside your cabin to cool air that is blown into the cabin after completing the closed-loop cycle.

Most low pressure AC hoses are made out of metal, with flexible rubber hose material for places where the hose needs to run through tight spots inside the engine compartment. Due to the fact that the engine compartment is extremely hot, the AC low pressure hose can sometimes develop small holes that causes refrigerant to leak and can render the AC system useless. If this occurs, you'll have to test the AC system for leaks to determine the precise location that is causing the AC system failure and replace those parts in order for the air conditioning in your vehicle to work smooth and correctly.

Part 1 of 4: Understanding the symptoms of a broken AC low pressure hose

When the low pressure side of the air conditioning system is damaged, symptoms tend to be noticed sooner than if the issue is on the high pressure side. This is due to the fact that the low pressure side is where the cool air is blown into the vehicle. When the leak is on the low pressure side, this means that the cabin is going to get less cool air. If the problem is with the high pressure hose, the symptoms will be not as noticeable at first.

Because the AC system on your vehicle is closed loop, it's critical for you to find the source of the leak, before making decisions on replacing parts. When the low pressure hose has a leak or is broken, it tends to display the following symptoms or warning signs.

There is a lack of cool air blowing into the cabin. When the low pressure hose has a leak, the first and most obvious sign is there will be less cool air flowing into the cabin. The low side is designed to carry the refrigerant to the compressor, so if there is a problem with the hose, it tends to negatively impact the entire AC system.

You see refrigerant building up on the hose. If you have a leak coming from the low pressure side of the AC system, it's very common for a greasy film to appear on the outside of the low pressure line. This is due to the fact that refrigerant flowing from this side of the AC system is gaseous. You'll typically find this on the fittings that secure the low pressure ac hoses to the compressor. If the leak is not fixed, eventually the refrigerant will drain and render the AC system completely useless. This can also cause other major parts of the AC system to fail.

You can hear refrigerant escaping from the pressure lines when you add refrigerant to the AC system. When there is a hole in the low pressure line itself, it's common for you to hear a hissing sound coming from underneath the car. There are two common ways to check for leaks at this point:

  • Place your hand over the hose and try to feel for the leaking refrigerant
  • Use a dye/refrigerant product that will display the source of the leak with the use of a UV or black light.

Part 2 of 4: Understanding what causes AC low pressure hoses to fail

For the most part, a low pressure hose failure will be caused by age, time and exposure to elements. It's very rare for the low pressure hose to become damaged. In fact, most AC leaks are caused seals on the AC compressor or the condenser that wear out, become cracked and cause refrigerant to leak from the system. If the refrigerant level goes too low, it's common for the AC compressor clutch to shut off automatically, disabling the system. This is done to reduce the potential of burning the compressor as the refrigerant is also used to cool the system.

When it comes to AC low pressure hose failures, the most common place the hose will fail is on the rubber parts of the hose or the connections to the other components. Most of the rubber parts of the hose are bent and can crack due to age or exposure to heat. Refrigerant is also corrosive and can cause the hose to rot from the inside of the hose until it develops a hole. Damage can also occur to the low pressure hose if there is too much AC refrigerant inside the system. This creates a situation where the hose itself can't handle the excess pressure and will either pop the seal where the hose attaches to the compressor or the hose will burst. This is a worst case scenario and not very common.

Part 3 of 4: Testing for an AC leak

Before you decide to replace the AC low pressure hose, you want to verify that the leak is coming from that particular component. As indicated above, most leaks come from seals on the AC compressor, the evaporator, the drier or the condenser. In fact, when you look at the diagram above, you'll see that on many AC systems, there are multiple low pressure hoses; connected from the compressor to the expansion valve and the expansion valve to the evaporator. Any of these hoses, connections or components can be the source of the refrigerant leak. This is a major reason why diagnosing air conditioning problems is a complex and time consuming process for even the most experienced mechanics.

However, there is a rather simple and cost-effective way to diagnose AC system leaks that a novice DIY mechanic can do on their own. In order to accomplish this test, you'll need to secure a few parts and materials first.

Materials Needed

  • Black light/UV light
  • Protective gloves
  • R-134 refrigerant with dye (one can)
  • Safety glasses
  • Schrader valve AC connector

Step 1: Raise the hood of your vehicle and prepare for service. To complete this test, you're going to follow similar steps that you would use to charge your AC system with a can of refrigerant. Each vehicle's system is unique, so refer to your own service manual for exact steps on how to charge the AC system.

For purposes of this article, we'll assume that your vehicle is charged from the low side port (which is most common).

Step 2: Locate the low side port of the AC system: On most domestic and foreign cars, trucks and SUV's the AC system is charged by attaching a Schrader valve connection to the port and to the can of refrigerant. Find the AC low side port, typically on the passenger side of the engine compartment and remove the cap (if there is one).

Step 3: Attach Schrader valve to the low side port. Make sure to attach the Schrader valve to the port by snapping the connection tightly. If the connection does not snap into place it's possible the low side port is damaged and possible the source of your leak.

Low side and high side ports are different sizes, so make sure you have the right type of Schrader valve connection for the low side port.

Once the valve is secured to the low pressure side port, attach the other end to the can of R-134 refrigerant/dye. Make sure the valve on the can is closed before installing the Schrader valve connection.

Step 4: Start the vehicle, turn on the AC system and activate the refrigerant canister. Once the can is secured to the valve, start your vehicle and let it heat up to operating temperature.

Then turn on your AC system to max cold setting and highest pressure setting. Let the AC system run for about 2 minutes, then turn the valve of the R-134 refrigerant/dye can to the "open" position.

Step 5: Activate the canister and add dye to the AC system. On your Schrader valve, you should have a pressure gauge that will display the pressure of the refrigerant. On most gauges, there will be a "green" section that is what tells you how much pressure to add to the system. With the can upside down (as recommended by most manufacturers) slowly turn the canister on until the pressure is in the green section or (the desired pressure as indicated by the dye's manufacturer).

The instructions on the can will tell you specifically how to check that the system is fully charged. However, most ASE certified mechanics will listen for the AC compressor to engage and run consistently for a period of 2 to 3 minutes. Once this happens, turn canister "off", shut the vehicle off and remove the Schrader valve attachment from the can and the low side valve.

Step 6: Use the black light to search for dye and leaks. Once the system has been charged and has cycled for about five minutes with the dye inside, the leaks can be detected by shining a black light (UV light) along all of the lines, and connections that comprise the AC system. If the leak is large, you'll be able to detect it easily. However, if it's a small leak, this process could take some time.

  • Tip: The best way to test for leaks using this method is in the dark. As crazy as it might sound, the UV light and dye shows up really well when it's completely dark. A good tip is to complete this test with as little light possible.

Once you find the dye exposed, use a drop light to illuminate the part so you can get a visual look at the part that is leaking. If the component that is leaking is coming from the low pressure hose, follow the steps in the next section for replacing the low pressure AC hose. If it's coming from another component, follow instructions in the service manual for your vehicle for replacing that part.

Part 4 of 4: Replacing the AC low pressure hose

Once you've diagnosed the source of the AC leak as being the low pressure hose, you'll have to order the right replacement parts and assemble the right tools to complete this repair. In order to replace hoses or any components of an AC system, you need to have specialized equipment to remove the refrigerant and pressure from the lines. Noted below are the materials and tools you'll need to complete this repair.

Materials Needed

  • AC manifold gauge kit
  • Empty refrigerant tank
  • End wrenches (variable sizes/check service manual)
  • Replacement low pressure hose
  • Replacement fittings (in some cases)
  • Replacement recommended refrigerant
  • Socket and Ratchet set
  • Safety glasses
  • Safety gloves
  • Vacuum pump and attachments for AC lines

  • Warning: The steps noted below are GENERAL STEPS for replacing the AC low pressure hose. Every AC system is unique for the manufactures, year, make and model. Always purchase and refer to your service manual for exact instructions on how to safely replace the low pressure hose of your AC system.

Step 1: Remove battery cables from positive and negative terminal. When replacing any mechanical components, it's always recommended to remove power from the battery. Remove the positive and negative cables from the terminal posts and make sure they don't connect to terminals during this repair.

Step 2: Complete procedures for draining refrigerant and pressure from your AC system. Once the battery cables have been removed, the first thing you'll need to do is remove pressure from the AC system.

There are multiple methods to complete this process, which is why it's always suggested to refer to your vehicle service manual. Most ASE certified mechanics will use an AC manifold gauge and vacuum system as displayed above to complete this step. In general, this process is completed using the steps below:

  • Connect the vacuum pump, manifold system and empty tank to the vehicle's AC system. On most kits, the blue lines will be attached to the low pressure fitting and the low side of the manifold gauge. The red fittings attach to the high side. The yellow lines attach to the vacuum pump, and a vacuum pump line is attached to an empty refrigerant tank.

  • Once all lines are secure, open all valves on the manifold, the vacuum pump and the empty tank.

  • Turn the vacuum pump on and let the system drain until the pressure gauges read ZERO on both the low and high pressure lines.

Step 3: Locate the leaking low pressure hose and replace. When you completed the pressure check in part three of this article, hopefully you marked which low pressure line was broken and needed to be replaced.

There are typically two different low pressure lines. The line that is commonly broken and constructed of rubber and metal is the line connected from the compressor to the expansion valve.

Step 4: Remove AC low pressure hose from expansion valve and compressor. The diagram above shows the connections where the low pressure lines are attached to the expansion valve. There are two common connections; the connection from this valve to the evaporator is typically all metal; so it's very rare that this is the source of your leak. The common connection is on the left of this image, where the AC low pressure hose attaches from the expansion valve to the compressor.

Follow your instructions located in the service manual, as each connection and fitting can be different for specific types of vehicles. However, the process for removing the low pressure line generally follows these steps:

  • The low pressure hose is removed from the compressor using an end wrench or line wrench.
  • The low pressure hose is then removed from the expansion valve
  • The new low pressure hose is run along the side of the vehicle and secured to clamps or fittings where the old hose was connected (refer to your service manual as this is always different for every vehicle).
  • The old low pressure hose is removed from the vehicle
  • The new low pressure hose is installed on the expansion valve
  • The new low pressure hose is secured to the compressor

Step 5: Check all AC low pressure hose connections: Once you've replaced the old hose with the new low pressure hose, you'll want to double check the connections to the compressor and the expansion valve. In many cases, the service manual will explain how to properly tighten the new connections. Make sure to double check that each fitting is secured as recommended by your manufacture. Failure to complete this step may result in refrigerant leaking.

Step 6: Recharge AC system. Charging an AC system after it's been completely drained is unique for each vehicle, so always refer to your service manual for instructions. The GENERAL STEPS are noted below using the same manifold system you used to drain the system.

  • Warning: Always use safety gloves and glasses when charging AC systems.

Locate the high and low side port. Most of the time they are colored blue (for low) and red (for high) or will have a cap with the letter "H" and "L."

  • Make sure all valves are closed before attaching
  • Attach manifold connections to the low and high side
  • Turn the valves on the Schrader valve secured to the ports to the fully "ON" position
  • Secure the vacuum pump and empty tank to the manifold
  • Turn on the vacuum pump to completely vacuum the system
  • Open the low and high side valves on the manifold and let the system vacuum check (this should be done for a minimum of 30 minutes).
  • Close low and high valves on the manifold and turn vacuum pump off
  • To check for leaks, leave the vehicle for 30 minutes with the lines still attached. If the manifold gauges are still in the same position, there are no leaks. If the gauge has increased, you still have a leak that need to be fixed.
  • Charge the AC system as a vapor (meaning make sure the tank is sitting down). Although this process is more time consuming, it is safer and reduces component damage.
  • Connect refrigerant canister to the manifold
  • Follow the instructions supplied by the service manual on how much refrigerant to add. It's also a good idea to use a refrigerant scale for consistency and accuracy.

  • Tip: You can also find the amount of refrigerant sometimes on the hood or the front clip of the engine compartment.

  • Open the canister valve and slowly loosen the center manifold connection to bleed air from the system. This purges the system.

  • Open low and high side manifold valves and let the refrigerant fill the system until the desired level is achieved. Using the scale method is really effective. Typically the refrigerant will stop flowing when the pressure inside the tank and the system is equal.

However, you need to start the vehicle and continue the filling process.

  • Close the high and low side valves before starting vehicle.

  • Start vehicle and turn on AC system to full – wait for the compressor clutch to engage or physically look at the compressor pump for it to activate.

  • ONLY open the low side valve to continue charging the system. Opening the high side valve will cause damage to the AC system.

  • Once the desired level has been achieved, close the low side valve on the manifold, shut off the tank, disconnect all fittings and place charge caps back onto the vehicle's AC system.

Once you've completed this process, the AC system should be fully charged and ready for years of service. As you can see, the process for changing the AC low pressure hose can be very complex and requires the use of specialty tools in order to correctly install the new line safely. If you've read these instructions and have determined that this might be too complex for you, contact one of our local ASE certified mechanics to complete the AC low pressure hose replacement 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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