Porsche AOS Replacement Parts: Everything You Need to Know

If you own a Porsche with a 3.6L or 3.8L flat-six engine—particularly the 996, 997, Boxster, or Cayman—you've likely heard about the AOS (Air Oil Separator). This component is critical for engine health, and when it fails, it can cause a range of issues. Replacing it with the right part is essential. This guide covers what the AOS does, signs of failure, replacement part options, and practical advice for choosing and installing a new one.

What Is the Porsche AOS and Why Does It Fail?

The Air Oil Separator (AOS) is a device that separates oil vapors from the crankcase ventilation gases. It returns oil to the sump and routes clean vapors into the intake to be burned. In Porsche engines, the AOS is especially prone to failure due to heat, age, and the design's sensitivity to oil consumption. Over time, the internal diaphragm can tear or the check valve can stick, causing oil to enter the intake system or vacuum leaks.

Symptoms of a failing AOS include:

• Excessive white smoke from exhaust (especially on startup)
• Oil in the intake or on air filter
• Rough idle or stalling
• Oil consumption increase
• Check engine light with lean codes (P0171, P0174)
• Hissing sound from the engine bay (vacuum leak)

Ignoring a bad AOS can lead to more serious problems like hydro lock (from oil pooling in intake), damaged catalytic converters, or engine damage from oil starvation.

OEM vs. Aftermarket Porsche AOS Replacement Parts

When replacing your AOS, you have two main categories: genuine Porsche (OEM) and aftermarket alternatives.

Genuine Porsche AOS

Original parts from Porsche are the most reliable choice. They come with a warranty and exact fitment. However, they are expensive—often $300–$500 for the part alone. The original design has known weaknesses, and some owners report that OEM replacements fail again after 30,000–50,000 miles.

Aftermarket AOS Options

Several aftermarket manufacturers produce AOS units for Porsche. Popular brands include:

• LN Engineering – Known for its "IMA" (Improved AOS) design that uses a metal housing and internal check valve. Many consider it an upgrade over OEM. Price ~$200–$300.
• Rennline – Offers billet aluminum AOS kits that eliminate plastic housings. These are rebuildable and often use replaceable internal filters. Price ~$250–$350.
• CRP (Custom Racing Products) – Produces a direct-fit aluminum AOS with improved flow. Price ~$200.

Aftermarket units often address OEM weaknesses by using metal instead of plastic, better internal seals, and sometimes added ports for catch cans. However, not all aftermarket parts are equal—some cheaper ones may not last. Stick with reputable brands.

Key Considerations When Choosing an AOS Replacement

Fitment

Make sure the AOS is designed for your specific model and engine. For example, 996 and 997 (non-turbo) use similar AOS units, but Boxster/Cayman (986/987) may require a variant. Turbo engines (996 Turbo, 997 Turbo) have a different AOS system. Confirm with the manufacturer.

Material

Plastic OEM units are prone to cracking at the vacuum nipple or where the lines connect. Aluminum or billet designs are more durable and resist heat better.

Rebuildability

Some aftermarket AOS units can be rebuilt by replacing a seal or internal element. This saves money long term.

Catch Can Integration

Many enthusiasts add a catch can between the AOS and intake to trap leftover oil mist. Some aftermarket AOS units have built-in ports for this. If you plan to run a catch can, consider a unit that supports it.

Installation Tips for Porsche AOS Replacement

Replacing the AOS is an intermediate-level DIY task. Expect to spend 2–4 hours. Here's an overview:

1. Access – On most 996/997, the AOS is located under the intake manifold on the left side (driver side on left-hand drive). You may need to remove the intake manifold, which requires disconnecting fuel lines, throttle body, and sensors.
2. Vacuum Lines – Carefully mark or label all vacuum hoses connected to the AOS. Many are brittle with age; plan to replace them.
3. Gaskets and O-rings – Replace any gaskets or O-rings that seal the AOS to the block or manifold. These are often included in aftermarket kits.
4. Check Valve – The old AOS may have a one-way check valve. Ensure your replacement has a similar function or you may create a vacuum leak.
5. Test – After installation, start the engine and check for smoke, vacuum leaks, and proper idle. Drive gently for a few miles and re-check.

If you're not comfortable working on your engine, a specialized Porsche shop can do the job in about 1–2 hours labor (typically $200–$400).

Cost of Porsche AOS Replacement Parts

• OEM Porsche AOS: $300–$500
• Aftermarket (reputable brands): $150–$350
• Install kit (gaskets, hoses): $30–$80
• Labor if professional: $200–$400

Total cost for a DIY replacement: around $200–$500 depending on the part chosen. Professional installation adds $200–$400.

Final Recommendation

For most Porsche owners, an aftermarket AOS from a reputable brand like LN Engineering or Rennline offers better durability and value than OEM. The upfront cost is similar or slightly lower, and you get an improved design that often outlasts the original. If you plan to keep the car long-term, a rebuildable aluminum unit is worth the investment. Always replace all associated vacuum lines and gaskets during the job to prevent future leaks. If you are not mechanically inclined, have a Porsche specialist do the work—improper installation can lead to engine damage.

By choosing the right AOS replacement part and installing it correctly, you can restore your Porsche's performance and avoid costly repairs down the road.