Audi S8 (2007-2018) Secondary Air Injection Control Valve Guide
That check engine light on your Audi S8 might be from a small but crucial emissions valve—here's the complete story.
- A check engine light with codes P0411, P0491, or P0492 is the primary symptom of a fault in the system.
- Before replacing this valve, you MUST rule out clogged carbon ports in the cylinder heads, a very common issue on the S8 4.0T engine.
- The control valve's failure to close allows moisture to destroy the air pump; they should often be inspected or replaced together.
- The vehicle is safe to drive with a bad valve, but it will not pass an emissions test.
Diagnosing a Faulty Air Control Valve in Your S8
When the secondary air injection system has a problem, your Audi S8 won't leave you stranded, but it will turn on the check engine light. Since this system is only for cold-start emissions, you likely won't notice any change in performance or fuel economy. The main issue is that you will not pass a state emissions test with the light on.
Symptoms of Failure
- Check Engine Light: This is the most common symptom. You will see codes like P0411 (Incorrect Flow), P0491 (Insufficient Flow, Bank 1), or P0492 (Insufficient Flow, Bank 2).
- Loud Noise on Cold Start: A sound like a vacuum cleaner or a small jet engine for the first minute after starting suggests the air pump is failing, often because the control valve is stuck open, allowing moisture into the pump.
- No Pump Sound: If you used to hear the pump and now it's silent, the pump motor has likely failed completely.
A Critical Warning for S8 Owners: Clogged Air Ports
For the Audi S8, especially the 2012-2018 D4 models with the 4.0T V8, a very common problem is clogged secondary air ports inside the cylinder heads. Carbon builds up over time and blocks the passages that the air travels through. This will cause the exact same P0491 and P0492 insufficient flow codes as a bad valve. Replacing the control valve or the air pump will NOT fix this issue. Diagnosing the system correctly is essential before buying parts. Audi has issued Technical Service Bulletins (TSBs) on this issue, which often requires a professional cleaning procedure.
Choosing the Right Aftermarket Control Valve
Since the control valve is a common failure point that can lead to the destruction of the more expensive air pump, replacing it with a quality part is important. When it comes to aftermarket options for your S8, brands fall into different tiers of quality and price.
- Hella: Often a direct supplier to Audi and other German automakers. Choosing Hella is the closest you can get to OEM quality without the dealership price tag. It is a top-tier choice for fit, function, and durability.
- Vaico: A well-regarded German aftermarket brand that produces a wide range of parts. Vaico is a solid mid-tier option that offers a good balance between cost and quality, making it a reliable choice for this repair.
- Dorman: A budget-friendly brand offering a cost-effective solution. While functional, it may not have the same long-term durability or precise finish as German-made OEM supplier brands.
| Part | Estimated Cost |
|---|---|
| New OEM Valve | $250 - $450 |
| New Aftermarket Valve | $13 - $148 |
| Shop Labor | $300 - $600 |
Pro Tip: Replace in Pairs
Your S8's V10 or V8 engine has two of these control valves—one for each cylinder bank. If one has failed due to age or getting stuck, the other is likely not far behind. To save on future labor costs, it is highly recommended to replace both valves at the same time.
Frequently Asked Questions (FAQ)
Can I just clean the old valve?
While it might be possible to temporarily free up a stuck valve with cleaners, it is not a reliable long-term fix. The internal mechanism is worn, and it will likely fail again. Given the high labor cost to access the valve, replacement is the recommended repair.
Where are the secondary air control valves located on the S8?
The control valves (combi valves) are mounted to the back of each cylinder head. Access is very tight, especially on the D4 S8 4.0T engine, and may require removing other components to reach 🎬 See this walkthrough for accessing components at the back of the engine. the bolts.
Why did my new air pump fail after a few months?
This is almost always caused by a faulty control valve that was not replaced along with the pump. If the valve sticks open, it allows hot exhaust and moisture to flow backward, quickly destroying the new pump. Always inspect or replace the control valves when replacing the air pump.
Is an aftermarket valve a good choice for my Audi?
Yes, as long as you choose a quality brand. An OEM-supplier brand like Hella provides the same performance as the dealer part for less money. Even a solid mid-tier brand is a smart choice over a non-functional valve, but proper diagnosis of the entire system is the most important step.
Helpful Videos
Technical Specifications
OEM Part Numbers: 079131102S 078131101AG 078131102T 07C131102D
The Secondary Air Injection Control Valve, or 'combi valve', is a one-way check valve that controls the flow of air from the SAI pump into the exhaust manifolds. On V-engines like the S8's V10 and V8, there are two valves—one for each cylinder bank. They can be operated by vacuum or pressure, depending on the specific system design.
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The information in this article is provided for general reference and educational purposes only. Vehicle specifications, procedures, and part compatibility can vary by production date, trim level, and region. Always consult your vehicle's factory service manual and verify part numbers before purchasing or performing repairs. Safety-critical components such as airbags, seat belts, and braking systems should be installed by a qualified professional.
- Audi S8: