Subaru Suspension Part with Two Plastic Tubes: Identification and Replacement Guide

If you own a Subaru Outback or Legacy equipped with an automatic load-leveling suspension, you may have come across a distinctive rear shock absorber that has two plastic tubes attached. This part is often referred to as the self-leveling shock or air-assisted shock. Understanding what this component does, when it fails, and how to replace it can save you time and money.

What Is the Subaru Suspension Part with Two Plastic Tubes?

The part in question is the rear shock absorber for Subaru models that feature an optional self-leveling rear suspension system. Unlike standard shocks, this unit incorporates two plastic tubes—one for air supply and one for venting or sensing—that connect to an air compressor or air spring system. The assembly is sometimes called a "shock with air line" or "self-leveling strut." It is most commonly found on Subaru Outback and Legacy models from the early 2000s through the 2010s, though some later models also use a similar design.

How the Self-Leveling System Works

Subaru's self-leveling system uses an air compressor mounted near the rear of the vehicle. When the rear of the car sags under load (such as when carrying heavy cargo or towing), the compressor inflates the air chambers within the rear shocks. This raises the ride height back to normal. The two plastic tubes serve as air lines: one delivers pressurized air from the compressor to the shock, and the other allows the system to vent air when the load is removed. A height sensor on the rear axle tells the compressor when to operate.

Signs That the Self-Leveling Shock Needs Replacement

Over time, the plastic tubes can become brittle and crack, or the internal seal in the shock can fail. Common symptoms include:

• Rear end sagging – The car sits lower in the back, especially when loaded.
• Air leaks – You may hear hissing sounds from the rear after parking.
• Rough ride – The rear suspension becomes stiff or bouncy.
• Check suspension light – Some models have a dashboard warning for suspension faults.
• Uneven tire wear – A sagging rear can change toe alignment.

If any of these occur, inspect the rear shock absorbers. Cracks or rubbing on the plastic tubes are a clear sign of failure. Sometimes the entire shock assembly must be replaced because the tubes are integrated.

Replacement Options for the Two-Tube Suspension Part

When it's time to replace the rear shocks with the two plastic tubes, you have two main paths:

OEM Replacement

Using genuine Subaru parts ensures exact fit and retains the self-leveling function. OEM shocks come with the tubes pre-attached and are designed to match the original performance. The downside is cost—OEM units are typically more expensive than aftermarket options. Also, availability can be an issue for older models.

Aftermarket Alternatives

Several aftermarket brands offer replacement self-leveling shocks that are compatible with Subaru's system. Some may use upgraded materials for the plastic tubes to reduce brittleness. Be sure to check that the aftermarket part includes the correct tube fittings and matches your vehicle's air line connector type. A few aftermarket options also allow you to replace only the shock while reusing the original tubes if they are still in good condition, but this is less common.

Coil Spring Conversion

Many owners choose to eliminate the self-leveling system entirely by installing standard coil-over shocks and springs from a non-self-leveling Subaru model. This conversion can be simpler and more reliable in the long run, as it removes the air compressor, tubes, and height sensor. The trade-off is that you lose the automatic load-leveling capability. For daily drivers that don't carry heavy loads often, this is a popular cost-effective solution. Kits are available that include all necessary hardware.

How to Choose the Right Replacement

Before purchasing, confirm that your Subaru actually has the self-leveling option. Check the window sticker, owner's manual, or look for the air compressor under the rear cargo area. Also verify the model year and engine type, as some variations exist. If you decide to stick with the original system, order only parts that list "self-leveling" or "air adjust" in the description.

• OEM part number – Look up the correct OEM part number for your specific model. This ensures the tube routing matches your vehicle.
• Aftermarket quality – Read the warranty and customer feedback on the brand. Some aftermarket shocks have a shorter lifespan than OEM.
• Conversion kit – If converting to coil springs, choose a reputable brand that includes springs, shocks, and hardware designed for your exact model.

Final Recommendation

For most Subaru owners, the most practical solution is to replace the failed self-leveling shock with an OEM part if the vehicle is relatively new and you frequently carry loads or tow. The self-leveling feature adds comfort and safety. However, if your Subaru is older, the conversion to standard coil-over suspension can be a more reliable and economical choice that eliminates future air line issues. Before buying, inspect the plastic tubes carefully—if only one tube is damaged, you might try replacing just the tube if the shock itself is still good. But in many cases, the entire shock assembly is the safest bet. Whichever route you take, addressing a failed self-leveling shock promptly prevents further strain on suspension components and keeps your Subaru handling as designed.